Revert "block: fix single range discard merge"

This reverts commit 42d72c9d.
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: I1f1c621b863a292e20dca6deef19802a25cf09a9
Signed-off-by: Sayali Lokhande <sayalil@codeaurora.org>