ext2: remove nobh support

The nobh mode is an obscure feature to save lowlevel for large memory
32-bit configurations while trading for much slower performance and
has been long obsolete.  Remove it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>