pool: Remove caching behaviour of file meta backend

Revert 8290f2f878a818a1398bea7c415a5b1274ec47eb

Tigran reported that the pool performs badly with the
simple caching behaviour added in the above patch.

Target: trunk
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5608/
(cherry picked from commit 98e8a9a34ea20a1eaae8f1c4ab5043b64e043360)