Remove :visited-link/Fenced Frame UMA

This histogram was added to get a rough understanding of how many links
were not being styled due to being in a Fenced Frame tree. I feel I have
gotten enough data from these to remove them now.

OBSOLETE_HISTOGRAMS=Only temporary to determine initial event frequency.

Change-Id: Ie9b5db1d455e2d52d02f3d1784d2ea7cc0af0c6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6212993
Commit-Queue: Kyra Seevers <kyraseevers@chromium.org>
Reviewed-by: David Baron <dbaron@chromium.org>
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1413038}