Revert of [mb] Switch staging bot to gyp for testing purposes (patchset #1...

Revert of [mb] Switch staging bot to gyp for testing purposes (patchset #1 id:1 of https://codereview.chromium.org/2056673002/ )

Reason for revert:
Looks good. Switch back to gn.

Original issue's description:
> [mb] Switch staging bot to gyp for testing purposes
>
> This is to test the migration workflow. Will be reverted
> afterwards.
>
> BUG=chromium:616035
> NOTRY=true
> TBR=vogelheim@chromium.org, jochen@chromium.org
>
> Committed: https://crrev.com/6a5d769be2b97a9ebca8ec58496cb20aed077f18
> Cr-Commit-Position: refs/heads/master@{#36848}

TBR=vogelheim@chromium.org,jochen@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=chromium:616035

Review-Url: https://codereview.chromium.org/2050243002
Cr-Commit-Position: refs/heads/master@{#36861}

infra/mb/mb_config.pyl

@@ -8,7 +8,7 @@
   # below). MB uses this dict to look up which config to use for a given bot.
   'masters': {
     'client.v8': {
-      'V8 Linux - swarming staging': 'gyp_swarming_release',
+      'V8 Linux - swarming staging': 'gn_swarming_release',
     },
   },