Clear the backlink cache on file delete

Clear the backlink cache on file delete

When a file is deleted pages that link to the file
(backlinks) are purged. The set of backlinks for a file
is cached in the backlink cache, and this is where the set
of backlinks that need purging is read from. If, at file delete
time, there is a backlink cache for the file but it is not
up to date, then backlinks missing from the set of cached
backlinks for that file will not be purged, leading to
broken links.

This patch clears the backlink cache on file delete before
initiating purging of backlinks.

Bug: T183478
Change-Id: I3bbd79e5a8fa14bf80ceee81e944108edada322e