checkpatch: Drop pr_warning check

All pr_warning are removed from kernel, let's cleanup pr_warning
check in checkpatch.

Link: http://lkml.kernel.org/r/20191128004752.35268-5-wangkefeng.wang@huawei.com
To: linux-kernel@vger.kernel.org
Cc: gregkh@linuxfoundation.org
Cc: tj@kernel.org
Cc: arnd@arndb.de
Cc: sergey.senozhatsky@gmail.com
Cc: rostedt@goodmis.org
Cc: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Joe Perches <joe@perches.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>