drop explicit disconnect in dtor

Summary:
when this is getting destroyed the qobject dtor will automatically
disconnect everything. the explicit disconnect calls serve no purpose and
aren't done in any other dtors.

Test Plan: it doesn't crash

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19684