[ticket/12747] Drop support for Firebird

PHPBB3-12747
[ticket/12747] Remove some methods which are not used anymore

PHPBB3-12747
Merge pull request #2629 from nickvergessen/ticket/12747

[ticket/12747] Drop support for Firebird

* nickvergessen/ticket/12747:
  [ticket/12747] Remove some methods which are not used anymore
  [ticket/12747] Drop support for Firebird

Conflicts:
	phpBB/phpbb/db/driver/firebird.php