MB-10921 Add a log message for couch_file delete

Added a log message when we delete a file. This will be helpful
to find which files are not getting cleanup after delete.

Change-Id: I9c79cfbd141edf05b1bb09918dd62d9d1e547dbf
Reviewed-on: http://review.couchbase.org/39233
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>