Remove page_size_t

MySQL 5.7 introduced the class page_size_t and increased the size of buffer pool page descriptors by introducing this object to them.

Maybe the intention of this exercise was to prepare for a future where the buffer pool could accommodate multiple page sizes. But that future never arrived, not even in MySQL 8.0. It is much easier to manage a pool of a single page size, and typically all storage devices of an InnoDB instance benefit from using the same page size.

Let us remove page_size_t from MariaDB Server. This will make it easier to remove support for ROW_FORMAT=COMPRESSED (or make it a compile-time option) in the future, just by removing various occurrences of zip_size.