Remove `unix_url` tool property.

Instead, we can just have `window_url` and the use the default
`url` as the fallback.

This simplification is part of the larger change to how we do
OS-specific properties.  See #1761

emsdk.py

diff --git a/emsdk.py b/emsdk.py
index ab023d8..436f701 100644
--- a/emsdk.py
+++ b/emsdk.py
@@ -1828,7 +1828,6 @@
   git_branch = None
   url = None
   macos_url = None
-  unix_url = None
   linux_url = None
   windows_url = None
 
@@ -1943,11 +1942,11 @@
       assert self.os in {'all', 'linux', 'win', 'macos'}
       if self.os == 'all':
         return True
-      if (WINDOWS and self.os == 'win') or (LINUX and (self.os in {'linux', 'unix'})) or (MACOS and (self.os in {'macos', 'unix'})):
+      if (WINDOWS and self.os == 'win') or (LINUX and self.os == 'linux') or (MACOS and self.os == 'macos'):
         return True
       return False
 
-    if not any((self.macos_url, self.windows_url, self.unix_url, self.linux_url)):
+    if not any((self.macos_url, self.windows_url, self.linux_url)):
       return True
 
     if MACOS and self.macos_url:
@@ -1959,9 +1958,6 @@
     if WINDOWS and self.windows_url:
       return True
 
-    if UNIX and self.unix_url:
-      return True
-
     return self.url is not None
 
   def get_version_file_path(self):
@@ -2083,8 +2079,6 @@
       return self.macos_url
     elif LINUX and self.linux_url:
       return self.linux_url
-    elif UNIX and self.unix_url:
-      return self.unix_url
     return self.url
 
   def install(self):

emsdk_manifest.json

diff --git a/emsdk_manifest.json b/emsdk_manifest.json
index dc54039..b1f880f 100644
--- a/emsdk_manifest.json
+++ b/emsdk_manifest.json
@@ -499,7 +499,7 @@
     "bitness": 32,
     "append_bitness": false,
     "windows_url": "https://github.com/emscripten-core/emscripten/archive/%tag%.zip",
-    "unix_url": "https://github.com/emscripten-core/emscripten/archive/%tag%.tar.gz",
+    "url": "https://github.com/emscripten-core/emscripten/archive/%tag%.tar.gz",
     "download_prefix": "emscripten-e",
     "activated_cfg": "EMSCRIPTEN_ROOT='%installation_dir%'",
     "activated_path": "%installation_dir%",
@@ -512,7 +512,7 @@
     "bitness": 64,
     "append_bitness": false,
     "windows_url": "https://github.com/emscripten-core/emscripten/archive/%tag%.zip",
-    "unix_url": "https://github.com/emscripten-core/emscripten/archive/%tag%.tar.gz",
+    "url": "https://github.com/emscripten-core/emscripten/archive/%tag%.tar.gz",
     "activated_cfg": "EMSCRIPTEN_ROOT='%installation_dir%'",
     "activated_path": "%installation_dir%",
     "activated_env": "EMSCRIPTEN=%installation_dir%",
@@ -522,7 +522,7 @@
     "id": "emscripten",
     "version": "%precompiled_tag%",
     "windows_url": "https://github.com/emscripten-core/emscripten/archive/%precompiled_tag%.zip",
-    "unix_url": "https://github.com/emscripten-core/emscripten/archive/%precompiled_tag%.tar.gz",
+    "url": "https://github.com/emscripten-core/emscripten/archive/%precompiled_tag%.tar.gz",
     "activated_cfg": "EMSCRIPTEN_ROOT='%installation_dir%'",
     "activated_path": "%installation_dir%",
     "activated_env": "EMSCRIPTEN=%installation_dir%"
@@ -533,7 +533,7 @@
     "bitness": 32,
     "append_bitness": false,
     "windows_url": "https://github.com/WebAssembly/binaryen/archive/%binaryen_tag%.zip",
-    "unix_url": "https://github.com/WebAssembly/binaryen/archive/%binaryen_tag%.tar.gz",
+    "url": "https://github.com/WebAssembly/binaryen/archive/%binaryen_tag%.tar.gz",
     "download_prefix": "binaryen-e",
     "activated_cfg": "BINARYEN_ROOT='%installation_dir%%generator_prefix%_64bit_binaryen'",
     "activated_path": "%installation_dir%%generator_prefix%_64bit_binaryen/bin",
@@ -549,7 +549,7 @@
     "bitness": 64,
     "append_bitness": false,
     "windows_url": "https://github.com/WebAssembly/binaryen/archive/%binaryen_tag%.zip",
-    "unix_url": "https://github.com/WebAssembly/binaryen/archive/%binaryen_tag%.tar.gz",
+    "url": "https://github.com/WebAssembly/binaryen/archive/%binaryen_tag%.tar.gz",
     "download_prefix": "binaryen-e",
     "activated_cfg": "BINARYEN_ROOT='%installation_dir%%generator_prefix%_64bit_binaryen'",
     "activated_path": "%installation_dir%%generator_prefix%_64bit_binaryen/bin",