Revert "Parser: Add Title type hints"

Revert "Parser: Add Title type hints"

This change reverts most of commit 3dff713fe4.

Especially the return type hints for getTitle() and Title() are reduced
to allow the type null as return value, because SematicMediaWiki uses
this by

	$this->parser->getTitle() instanceof Title

to check if there is a valid Title object.

The parameter type hints for setTitle() and Title() are kept.

Bug: T235392
Change-Id: I72ac1c9d37059876dbc7cd38158e7abd212da8fe
(cherry picked from commit d91a136ae7b53f07bac62aba944f5a7ba1ccd7ec)