Revert "Add the possibility to output a json gtest output to the perf tests."

This reverts commit cbfa235b3548ae1a0b8f5b80a879214ca5f1bd56.

Reason for revert: iOS bots will use flags_compatibility with isolated_script_test_output but not gtest_output.

Original change's description:
> Add the possibility to output a json gtest output to the perf tests.
>
> We use the Chromium existing flag isolated_script_test_output that we translate into gtest_output.
> This is because the Chromium flag has the same purpose as gtest_output and is already provided in the recipe modules.
>
> No-Presubmit: True
> Bug: b/197492097
> Change-Id: Ia432a85b0e0ab32008b39ffe751d11aefb9b24ea
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/251041
> Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
> Commit-Queue: Jeremy Leconte <jleconte@google.com>
> Cr-Commit-Position: refs/heads/main@{#35937}

No-Presubmit: True
Bug: b/197492097
Change-Id: I94e75328570f89011fbb0daf035f0072b8ea2f7d
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/252040
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#36042}
diff --git a/tools_webrtc/flags_compatibility.py b/tools_webrtc/flags_compatibility.py
index e7aeb93..72c66a5 100755
--- a/tools_webrtc/flags_compatibility.py
+++ b/tools_webrtc/flags_compatibility.py
@@ -22,7 +22,9 @@
 
   test_command = _ForcePythonInterpreter(unrecognized_args)
   if args.isolated_script_test_output:
-    test_command += ['--gtest_output=json:' + args.isolated_script_test_output]
+    test_command += [
+        '--isolated_script_test_output', args.isolated_script_test_output
+    ]
   if args.isolated_script_test_perf_output:
     test_command += [
         '--isolated_script_test_perf_output=' +