Remove use_rbe gn arg support

As it is removed in https://crrev.com/c/3744967, this cleans up the check for a no longer existing arg.

Bug: chromium:1247781
Change-Id: Ic7288cf34e63cd52087e2b51f1b4c816bbbc0363
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3745201
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Richard Wang <richardwa@google.com>

autoninja.py

@@ -82,8 +82,7 @@ def main(args):
   if os.path.exists(os.path.join(output_dir, 'args.gn')):
     with open(os.path.join(output_dir, 'args.gn')) as file_handle:
       for line in file_handle:
-        # Either use_goma, use_remoteexec or use_rbe (in deprecation)
-        # activate build acceleration.
+        # Either use_goma or use_remoteexec will activate build acceleration.
         #
         # This test can match multi-argument lines. Examples of this are:
         # is_debug=false use_goma=true is_official_build=false
@@ -95,7 +94,7 @@ def main(args):
                      line_without_comment):
           use_goma = True
           continue
-        if re.search(r'(^|\s)(use_rbe|use_remoteexec)\s*=\s*true($|\s)',
+        if re.search(r'(^|\s)(use_remoteexec)\s*=\s*true($|\s)',
                      line_without_comment):
           use_remoteexec = True
           continue