erofs: get rid of `z_erofs_next_pcluster_t`

It was originally intended for tagged pointer reservation.

Now all encoded data can be represented uniformally with
`struct z_erofs_pcluster` as described in commit bf1aa039
("erofs: sunset `struct erofs_workgroup`"), let's drop it too.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250114034429.431408-2-hsiangkao@linux.alibaba.com