Remove workarounds for `plugin_lint_mac` needed for older version of Cocoapods (#140395)

CI has been updated to Cocoapods 1.13, which fixes previous issues with the `plugin_lint_mac` test. Remove workarounds added previously.

Fixes https://github.com/flutter/flutter/issues/125812. 

See also https://github.com/flutter/flutter/issues/133584.

dev/devicelab/bin/tasks/plugin_lint_mac.dart

@@ -39,9 +39,6 @@ Future<void> main() async {
             iosintegrationTestPodspec,
             '--use-libraries',
             '--verbose',
-            // TODO(cyanglaz): remove allow-warnings when https://github.com/flutter/flutter/issues/125812 is fixed.
-            // https://github.com/flutter/flutter/issues/125812
-            '--allow-warnings',
           ],
         );
 
@@ -50,9 +47,6 @@ Future<void> main() async {
           macosintegrationTestPodspec,
           <String>[
             '--verbose',
-            // TODO(cyanglaz): remove allow-warnings when https://github.com/flutter/flutter/issues/125812 is fixed.
-            // https://github.com/flutter/flutter/issues/125812
-            '--allow-warnings',
           ],
         );
       });
@@ -529,8 +523,6 @@ Future<void> _tryMacOSLint(
   String podspecPath,
   List<String> extraArguments,
 ) async {
-  final StringBuffer lintStdout = StringBuffer();
-  try {
   await eval(
     'pod',
     <String>[
@@ -539,17 +531,5 @@ Future<void> _tryMacOSLint(
       podspecPath,
       ...extraArguments,
     ],
-      stdout: lintStdout,
   );
-  } on BuildFailedError {
-    // Temporarily ignore errors due to DT_TOOLCHAIN_DIR if it's the only error.
-    // This error was introduced with Xcode 15. Fix was made in Cocoapods, but
-    // is not in an official release yet.
-    // TODO(vashworth): Stop ignoring when https://github.com/flutter/flutter/issues/133584 is complete.
-    final String lintResult = lintStdout.toString();
-    if (!(lintResult.contains('error: DT_TOOLCHAIN_DIR cannot be used to evaluate') &&
-        lintResult.contains('did not pass validation, due to 1 error'))) {
-      rethrow;
-    }
-  }
 }