f2fs: delete obsolete FI_DROP_CACHE

[ Upstream commit bb6e1c8f ]

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>
Stable-dep-of: 54607494 ("f2fs: compress: fix to avoid inconsistence bewteen i_blocks and dnode")
Signed-off-by: Sasha Levin <sashal@kernel.org>