checkpatch.pl: reduce obsolete checks

script/checkpatch.pl seems to be imported from linux kernel directly,
so it contains lots of obsolete checks only for linux source tree.

This patch reduces clearly obsolete checks and make the script a
little bit faster (e.g. with this patch, checking the 4th patch of the
shephered (near 2000 lines) can be executed about 0.1 second faster)
and cleaner.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <tailai.ly@taobao.com>