f2fs: merge lz4hc_compress_pages() to lz4_compress_pages()

Remove unnecessary lz4hc_compress_pages().

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
[Jaegeuk Kim: clean up]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>