Remove error upgrades (dart-lang/stream_transform#118)

These are not necessary with `--fatal-infos`.
diff --git a/pkgs/stream_transform/analysis_options.yaml b/pkgs/stream_transform/analysis_options.yaml
index 90cc2c2..39d5c5e 100644
--- a/pkgs/stream_transform/analysis_options.yaml
+++ b/pkgs/stream_transform/analysis_options.yaml
@@ -6,11 +6,6 @@
     strict-raw-types: true
   errors:
     todo: ignore
-    dead_code: error
-    override_on_non_overriding_method: error
-    unused_element: error
-    unused_import: error
-    unused_local_variable: error
 linter:
   rules:
     - annotate_overrides