GitLab

This removes merge session stage for auth context preparation once it
is deprecated and source of extra failures.

TEST=Locally
BUG=b:148670963

Change-Id: I61fafb236a83cf2b9e9f1f65cd8ae029c734fe3c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2048109Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Kush Sinha <sinhak@chromium.org>
Commit-Queue: Yury Khmel <khmel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740305}