Revert "block: fix the DISCARD request merge"

This reverts commit 10807b37.
This is needed to fix regression observed in reboot path,
where discard requests are merged and for some reason merged
request is not properly freed or cleaned up resulting in panic
in __blk_put_request().

[ 338.244938] Call trace:
[ 338.244940] __blk_put_request+0x310/0x318
[ 338.244943] blk_queue_bio+0x170/0x698
[ 338.244946] generic_make_request+0x1f8/0x420
[ 338.244949] submit_bio+0x140/0x1d8
[ 338.244953] __submit_discard_cmd+0x3c4/0x4e8
[ 338.244956] __issue_discard_cmd+0x1c4/0x480
[ 338.244960] f2fs_issue_discard_timeout+0x64/0x128
[ 338.244962] f2fs_put_super+0x88/0x2c8
[ 338.244966] generic_shutdown_super+0x70/0xf8
[ 338.244968] kill_block_super+0x2c/0x60
[ 338.244972] kill_f2fs_super+0xb4/0xe0.

Change-Id: I585d54ca6be67eb31062f2de44472b27792320d1
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>