ext4: remove single extent cache

Single extent cache could be removed because we have extent status tree
as a extent cache, and it would be better.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: Jan kara <jack@suse.cz>

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

diff --cc fs/ext4/ext4_extents.h
Simple merge

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

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

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