HistoryServiceObserver: Rename OnURLsDeleted to OnHistoryDeletions

OnURLsDeleted was always a bit of a misnomer because DeletionInfo could
specify a lot more detail than which URLs were deleted.

In this recent CL, we made DeletionInfo also contain which visits were
deleted, so it's become even more of a misnomer.

This CL renames OnURLsDeleted to the more general name:
OnHistoryDeletions.

Bug: 329495955
Change-Id: I386585ac165d4806b4489cb6980fdb1c0ec60946
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5372508
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1278297}