LSM: Remove security_task_create() hook.

Since commit a79be238 ("selinux: Use task_alloc hook rather than
task_create hook") changed to use task_alloc hook, task_create hook is
no longer used.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <james.l.morris@oracle.com>