f2fs: delete obsolete FI_FIRST_BLOCK_WRITTEN

Commit 3c6c2beb ("f2fs: avoid punch_hole overhead when releasing
volatile data") introduced FI_FIRST_BLOCK_WRITTEN as below reason:

This patch is to avoid some punch_hole overhead when releasing volatile
data. If volatile data was not written yet, we just can make the first
page as zero.

After commit 7bc155fe ("f2fs: kill volatile write support"), we
won't support volatile write, but it missed to remove obsolete
FI_FIRST_BLOCK_WRITTEN, delete it in this patch.

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