nfsd: remove redundant test from nfsd_reply_cache_free

Entries can only get a c_type of RC_REPLBUFF iff they are
RC_DONE. Therefore the test for RC_DONE isn't necessary here.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>