f2fs: remove redundant sanity check in sanity_check_inode()

Commit f240d3aa ("f2fs: do more sanity check on inode") missed
to remove redundant sanity check on flexible_inline_xattr flag, fix
it.

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