LU-12511 fld: remove unused functions from fld_cache.c

LU-12511 fld: remove unused functions from fld_cache.c

fld_cache_delete and fld_cache_entry_lookup{,nolock}
are never used, so they can be removed.
fld_cache_entry_delete is only used locally to fld_cache.c
so it can be made static.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: Ied370420c348c95e14897187daaaec6708b5c96a
Reviewed-on: https://review.whamcloud.com/37737
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>