Revert "We automatically load classes now so no need to require."

This reverts commit 3ab636f75fc763f8103de5b90145d523a418c7f8.

The MantisCorePlugin class won't autoload using the autoloader in
core.php because this class is within MantisPlugin.class.php

Thus the commit being reverted here essentially prevented any plugin
from loading as the MantisCorePlugin dependency couldn't be met.