trigger_scripts: remove unnecessary code path

It seems --shard-index is always specified now.

This is partial re-land of https://crrev.com/c/2639441

Bug: 1127205
Cq-Include-Trybots: luci.chromium.try:chromeos-kevin-rel
Change-Id: If26fc49d61581635ae9260805d982768e285823a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640526
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845716}

testing/trigger_scripts/chromeos_device_trigger.py

@@ -65,18 +65,10 @@ def parse_args(triggerer):
       dest='optional_dimensions',
       help='Optional dimensions which will result in additional task slices. '
            'Duplicated from the `swarming.py trigger` command.')
-  # BaseTestTriggerer's setup_parser_contract() takes care of adding needed
-  # swarming.py args if they're not already present. But only do this if
-  # '--shard-index' is passed in. (The exact usage of trigger scripts are
-  # currently changing. See crbug.com/926987 for more info.)
-  if '--shard-index' in sys.argv:
-    base_test_triggerer.BaseTestTriggerer.setup_parser_contract(parser)
-    args, additional_args = parser.parse_known_args()
-    additional_args = triggerer.modify_args(
-        additional_args, 0, args.shard_index, args.shards, args.dump_json)
-  else:
-    base_test_triggerer.BaseTestTriggerer.add_use_swarming_go_arg(parser)
-    args, additional_args = parser.parse_known_args()
+  base_test_triggerer.BaseTestTriggerer.setup_parser_contract(parser)
+  args, additional_args = parser.parse_known_args()
+  additional_args = triggerer.modify_args(
+      additional_args, 0, args.shard_index, args.shards, args.dump_json)
 
   if additional_args[0] != 'trigger':
     parser.error(