Our benchmarking efforts indicate that the reasons for splitting the
buf_pool in commit c18084f7
have mostly gone away, possibly as a result of
mysql/mysql-server@ce6109ebfdedfdf185e391a0c97dc6d33867ed78
or similar work.

Only in one write-heavy benchmark where the working set size is
ten times the buffer pool size, the buf_pool->mutex would be
less contended with 4 buffer pool instances than with 1 instance,
in buf_page_io_complete(). That contention could be alleviated
further by making more use of std::atomic and by splitting
buf_pool_t::mutex further (MDEV-15053).

We will deprecate and ignore the following parameters:

	innodb_buffer_pool_instances
	innodb_page_cleaners

There will be only one buffer pool and one page cleaner task.

In a number of INFORMATION_SCHEMA views, columns that indicated
the buffer pool instance will be removed:

	information_schema.innodb_buffer_page.pool_id
	information_schema.innodb_buffer_page_lru.pool_id
	information_schema.innodb_buffer_pool_stats.pool_id
	information_schema.innodb_cmpmem.buffer_pool_instance
	information_schema.innodb_cmpmem_reset.buffer_pool_instance