f2fs: remove unused PAGE_PRIVATE_ATOMIC_WRITE

Commit 3db1de0e ("f2fs: change the current atomic write way")
has removed all users of PAGE_PRIVATE_ATOMIC_WRITE, remove its
definition and related functions.

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