ext2: add warning when specifying nocheck option

The option nocheck(nocheck/check=none) is useless but considering
backwards compatibility it's better to print warning for a while
before completely remove from the code.

This patch add proper warning message for option 'nocheck' and
remove unnecessary comment/function declaration which is used for
removed option 'check'.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Signed-off-by: Jan Kara <jack@suse.cz>