tomoyo: remove unused function declaration

The last usage of tomoyo_check_flags() has been removed by commit
57c2590f ("TOMOYO: Update profile structure."). Clean up its
residual declaration.

Signed-off-by: GONG, Ruiqi <gongruiqi1@huawei.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>