Revert "f2fs: let discard thread wait a little longer if dev is busy"

This reverts commit 161d1c91.

This is a preparation change for merging android-4.14 commit
73421a43 into msm-4.14 branch.
The reverted change is committed already as:

39fe1cda f2fs: let discard thread wait a little longer if dev is busy

Change-Id: Id68c93b211c9c5a1de850e40591a82f4cc1c74ef
Signed-off-by: Blagovest Kolenichev <bkolenichev@codeaurora.org>