[build] Remove obsolete gyp flag

This was removed on the infra side:
https://crrev.com/c/912831

TBR=sergiyb@chromium.org

Bug: chromium:772804
Change-Id: Icf0a33c2c1294edbc957c7e0e4aa78268997ae10
Reviewed-on: https://chromium-review.googlesource.com/913210Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51233}

tools/mb/mb.py

@@ -99,8 +99,6 @@ class MetaBuildWrapper(object):
                         dest='isolate_map_files')
       subp.add_argument('-g', '--goma-dir',
                         help='path to goma directory')
-      # TODO(machenbach): Remove after removing on infra-side.
-      subp.add_argument('--gyp-script', help='Deprecated')
       subp.add_argument('--android-version-code',
                         help='Sets GN arg android_default_version_code')
       subp.add_argument('--android-version-name',