[bookmarks] Clean up metrics related to batch upload

The main purpose of these metrics was to safely roll out a new
implementation of the batch upload logic. This is now complete and the
old code cleaned up, so there are no good reasons to keep these metrics
around for longer.

Change-Id: I59f9efa363951b7aaf4b7b3821f680e933235666
Bug: 332532186
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6415649
Reviewed-by: Maksim Moskvitin <mmoskvitin@google.com>
Commit-Queue: Mikel Astiz <mastiz@chromium.org>
Reviewed-by: Emily Shack <emshack@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1441508}