branches/zip: Non-functional change: Tighten debug assertions and remove dead code.
branches/zip: Non-functional change: Tighten debug assertions and
remove dead code.

buf_flush_ready_for_flush(), buf_flush_try_page(): Assert that
flush_type is one of BUF_FLUSH_LRU or BUF_FLUSH_LIST.  The flush_type
comes from buf_flush_batch(), which already asserts this.  The
assertion holds for all calls in the source code.

buf_flush_try_page(): Remove the dead case BUF_FLUSH_SINGLE_PAGE
of switch (flush_type).

davidl-zend/zenddbi,slanterns/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,ollie314/server,flynn1973/mariadb-aix,ollie314/server,ollie314/server,ollie314/server,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,davidl-zend/zenddbi,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server,davidl-zend/zenddbi,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,ollie314/server,flynn1973/mariadb-aix,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,ollie314/server