disk_cache: Remove redundant LastUsedTime from SqlEntryImpl

SqlEntryImpl::LastUsedTime() was redundant as SqlEntryImpl already
implements the disk_cache::Entry interface, which provides GetLastUsed().

This change removes LastUsedTime() and updates its only call site in
SqlBackendImpl to use GetLastUsed() instead. This simplifies the
SqlEntryImpl interface and promotes using the base class methods.

Bug: 422065015
Change-Id: I21ac79bcee0446e0d20925004ee348cc7e94b28c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6726279
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Mingyu Lei <leimy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1486140}