Remove the keychain argument.

It's not used any more, and the calling script was changed
to no longer provide it.

Bug: 913074
Change-Id: I01250b226eddb40fd4d523c75f9132c97629a8be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904232
Auto-Submit: Avi Drissman <avi@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713577}

chrome/installer/mac/sign_chrome.py

@@ -80,9 +80,6 @@ def create_config(config_args, development):
 def main():
     parser = argparse.ArgumentParser(
         description='Code sign and package Chrome for channel distribution.')
-    parser.add_argument(
-        '--keychain',
-        help='The keychain to load the identity from. OBSOLETE AND INGORED.')
     parser.add_argument(
         '--identity',
         required=True,