Remove --isolated from mb.py

This flag is not supported now.

Bug: chromium:1271841
Change-Id: I6a80ebed79a03164f58f1757b8ebaca74a681441
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3306786
Auto-Submit: Junji Watanabe <jwata@google.com>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78198}

tools/mb/mb.py

@@ -804,8 +804,6 @@ class MetaBuildWrapper(object):
     self.WriteJSON(
       {
         'args': [
-          '--isolated',
-          self.ToSrcRelPath('%s/%s.isolated' % (build_dir, target)),
           '--isolate',
           self.ToSrcRelPath('%s/%s.isolate' % (build_dir, target)),
         ],