Remove useless $out parameter from SkinTemplate::prepareQuickTemplate()

Remove useless $out parameter from SkinTemplate::prepareQuickTemplate()

Fix for I0a9a7f10ea (f5a0c23); use $this->getOutput() instead.

Passing different OutputPage object to Skin methods than the one set in
the context is deprecated, so there is no point having that parameter.

The method's signature is also misleading, since it allows null to be
passed, but if that value would be passed to it, the code would
crash since a such case is not handled in the method.

The $out parameter passed to the method from outputPage() is left since
it is required by the MobileFrontend extension. A similar commit will
be done in the extension, and the parameter's removal will happen once
this condition is lifted.

Change-Id: I4fac5b14afeab25f6f79f889c7ce56874827717d