Remove checks formerly for MySQL server version

Remove checks formerly for MySQL server version

Removed these checks instead of fixing them, for the stated reasons:

* initEditCount.php: "instanceof DatabaseMysql" is not a correct way to
  check if the database server is MySQL/MariaDB, because DatabaseMysql is
  now a subclass of DatabaseMysqlBase, only used as a wrapper around the
  deprecated mysql PHP extension (not the newer mysqli extension). The
  check was intended for pre-4.1 versions of MySQL, which do not support
  subqueries.

  The effect of this change is to use single-query mode whenever there is
  only one configured database server, even when using the mysqli PHP
  extension, which would have been the original intent.

* fixBug20757.php: If the database server is not MySQL/MariaDB, the
  $lowerLeft variable would remain undefined. The script was only intended
  for use with MySQL, so the check was only necessary in order to support
  pre-4.1 versions of MySQL, which require different SQL.

Follows-up b74f88967bf090af.

Change-Id: I7f32aed4473e5ea39dc40449ddc0af5f9a10df16