ceph: remove unused and redundant variable dropping

Variable dropping is set but never read and hence is redundant
and can be removed. Cleans up clang warning:

  fs/ceph/caps.c:1170:2: warning: Value stored to 'dropping' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>