f2fs: don't check PAGE_SIZE again in sanity_check_raw_super()

Many flash devices read and write a single IO based on a multiple
of 4KB, and we support only 4KB page cache size now.

Since we already check page size in init_f2fs_fs(), so remove page
size check in sanity_check_raw_super().

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Shaohua Liu <liush@allwinnertech.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>