Reverts "Use more CORS headers for flutter run server (flutter#152048)" (flutter#152248)

Reverts: flutter#152048
Initiated by: cbracken
Reason for reverting: breaks tool tests.

Example:
```
00:06 +762 ~2 -1: test/general.shard/web/web_asset_server_test.dart: release asset server serves correct mime type and content length for png [E]
  Expected: {'Content-Type': 'image/png', 'content-length': '64'}
    Actual: {
              'Content-Type': 'image/png',
              'Cross-Origin-Resource-Policy': 'cross-origin',
              'Access-Contr
Original PR Author: johnpryan

Reviewed By: {eyebrowsoffire}

This change reverts the following previous change:
@eyebrowsoffire

Author: auto-submit[bot]

packages/flutter_tools/lib/src/isolated/devfs_web.dart

@@ -1192,9 +1192,7 @@ class ReleaseAssetServer {
               'application/octet-stream';
       return shelf.Response.ok(bytes, headers: <String, String>{
         'Content-Type': mimeType,
-        'Cross-Origin-Resource-Policy': 'cross-origin',
-        'Access-Control-Allow-Origin': '*',
-        if (_needsCoopCoep && _fileSystem.path.extension(file.path) == '.html') ...<String, String>{
+        if (_needsCoopCoep && file.basename == 'index.html') ...<String, String>{
           'Cross-Origin-Opener-Policy': 'same-origin',
           'Cross-Origin-Embedder-Policy': 'require-corp',
         }