erofs: get rid of `struct z_erofs_collector'

Avoid `struct z_erofs_collector' since there is another context
structure called "struct z_erofs_decompress_frontend".

No logic changes.

Link: https://lore.kernel.org/r/20220301194951.106227-1-hsiangkao@linux.alibaba.com
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>