erofs: get rid of `enum z_erofs_page_type'

Remove it since pagevec[] is no longer used.

Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220715154203.48093-10-hsiangkao@linux.alibaba.com