f2fs: compress: remove unneeded code

- f2fs_write_multi_pages
 - f2fs_compress_pages
  - init_compress_ctx
  - compress_pages
  - destroy_compress_ctx  --- 1
 - f2fs_write_compressed_pages
 - destroy_compress_ctx  --- 2

destroy_compress_ctx() in f2fs_write_multi_pages() is redundant, remove
it.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>