Remove Cirrus check for web test concurrency (#143046)

Tests no longer run on Cirrus.
#40630

dev/bots/test.dart

@@ -2285,8 +2285,6 @@ Future<void> _runFlutterWebTest(String webRenderer, String workingDirectory, Lis
     flutter,
     <String>[
       'test',
-      if (ciProvider == CiProviders.cirrus)
-        '--concurrency=1',  // do not parallelize on Cirrus, to reduce flakiness
       '-v',
       '--platform=chrome',
       '--web-renderer=$webRenderer',
@@ -2482,21 +2480,6 @@ void adjustEnvironmentToEnableFlutterAsserts(Map<String, String> environment) {
   environment['FLUTTER_TOOL_ARGS'] = toolsArgs.trim();
 }
 
-enum CiProviders {
-  cirrus,
-  luci,
-}
-
-CiProviders? get ciProvider {
-  if (Platform.environment['CIRRUS_CI'] == 'true') {
-    return CiProviders.cirrus;
-  }
-  if (Platform.environment['LUCI_CONTEXT'] != null) {
-    return CiProviders.luci;
-  }
-  return null;
-}
-
 /// Checks the given file's contents to determine if they match the allowed
 /// pattern for version strings.
 ///