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/+/3745201Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Bruce Dawson <brucedawson@chromium.org>
Commit-Queue: Richard Wang <richardwa@google.com>