Avoid deleting ourselves midway through updateShadow

Summary:
If updating a shadow means that there should be no shadow, shadow ends
up deleting itself midway through the method.

It's cleaner and safer to leave that to the caller.

This new change matches the existing documentation for
Shadow::updateShadow which states

In case the Property has been withdrawn the method returns @c false.
In that case the owner should delete the Shadow.

Test Plan:
Created an ASAN of kwin
Relevant testMaximised no longer fails

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D17380