[BTM] Remove empty entries from `bounces` table in BTM DB.

https://crrev.com/c/6932843 performed a database migration from v9 to
v10 that dropped columns from the `bounces` table. However, the CL
failed to clear the newly empty rows from the DB. This resulted in a
crash (http://crash/b764d17e5e7f13a1) because
content::BtmStorage::ReadSite in browser/btm/btm_storage.cc performs a
check for entries without timestamps.

This CL introduces a new DB migration to remove entries that are newly
empty as a result of the previous migration that dropped columns in
addition to the changes introduced in https://crrev.com/c/6932843.

Bug: 444652236
Change-Id: Icf96d0ac2023ad7c53725b5c938ee9e2334da293
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6943731
Reviewed-by: Svend Larsen <svend@chromium.org>
Reviewed-by: Andrew Liu <liu@chromium.org>
Commit-Queue: Andrew Liu <liu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1515081}