Deleting the about dialog when it gets closed

Summary:
We are providing the parent if it is a QWidget, but still,
there is no need to have this window remain in memory
after closing.

If the parent is not a QWidget, this is a memory leak.

Reviewers: mart, Plasma, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2549