erofs: get rid of `struct z_erofs_collection'

erofs: get rid of `struct z_erofs_collection'

It was incompletely introduced for deduplication between different
logical extents backed with the same pcluster.

We will have a better in-memory representation in the next release
cycle for this, as well as partial memory folios support. So get rid
of it instead.

No logic changes.

Link: https://lore.kernel.org/r/20220529055425.226363-2-xiang@kernel.org
Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>