f2fs: remove wbc->for_reclaim handling

Since commits 7ff0104a ("f2fs: Remove f2fs_write_node_page()") and
3b47398d ("f2fs: Remove f2fs_write_meta_page()'), f2fs can't be
called from reclaim context any more.  Remove all code keyed of the
wbc->for_reclaim flag, which is now only set for writing out swap or
shmem pages inside the swap code, but never passed to file systems.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>