Remove unnecessary severity changes (#221)

These had been put in when there were specific warnings treated as fatal
in google3. Since we now treat infos as fatal its not necessary to
change the severity of individual warnings.
diff --git a/analysis_options.yaml b/analysis_options.yaml
index 72e4a38..d12202a 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -1,10 +1,4 @@
 include: package:lints/recommended.yaml
-analyzer:
-  errors:
-    unused_element: error
-    unused_import: error
-    unused_local_variable: error
-    dead_code: error
 linter:
   rules:
     # Errors