[CodeHealth] Remove unused functions in MainThreadEventQueue


Removed unused a few functions and variables in MainThreadEventQueue
They were used for recording metrics and metrics were removed a while
ago.

Change-Id: I724ff6a4b80d34f7a0b9de0db2376f5fa35eb6be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1946480
Reviewed-by: Navid Zolghadr <nzolghadr@chromium.org>
Commit-Queue: Ella Ge <eirage@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720881}
1 file changed