f2fs: delete obsolete FI_DROP_CACHE

FI_DROP_CACHE was introduced in commit 1e84371f ("f2fs: change
atomic and volatile write policies") for volatile write feature,
after commit 7bc155fe ("f2fs: kill volatile write support"),
we won't support volatile write, let's delete related codes.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>