ext4: remove vestiges of nobh support

The nobh option was only supported for writeback mode, but given that all
write paths actually create buffer heads it effectively was a no-op already.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

diff --cc fs/ext4/ext4.h
Simple merge

diff --cc fs/ext4/ext4_jbd2.h
Simple merge

diff --cc fs/ext4/inode.c
Simple merge

diff --cc fs/ext4/super.c
Simple merge