security: remove unused cap_is_fs_cap function

Since commit 3bc1fa8a ("LSM: remove BSD secure level security module")
there is no user of cap_is_fs_cap any more, so remove it.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>