rdbms: Simplify Database::factory()

rdbms: Simplify Database::factory()

Follows-up 0ff2b7a776 (T120333) which removed support for the
deprecated php 'mysql' extension.

It surprised me that the 'DatabaseMysql' entry could safely be removed
from $classAliases. This was because the key is not direct input,
but based on $canonicalDBTypes, which maps mysql to mysqli.

Take this further by removing the indirection between type, driver
and class name.

* Map built-in db types directly to their class and driver.
* Change fallback to assign $class directly from $dbType, without
  indirection of a $driver variable.
* Remove unused $classAliases.

Change-Id: I1200b07f66f23624410d848b6f382cb2dafa5c59