Fix a crash after removing a vector layer with active selection of shapes

Steps to reproduce:

0. set undo limit to 10 steps;
1. add a vector layer;
2. add a lot of shapes/text to that;
3. select some of these shapes
4. remove the vector layer;
5. do 10 brush strokes to make sure the vector layer is pushed out of the history;

The problem happened in the following way:

1. On destruction of the layer, KoSelection::deselect() was called
2. This deselect() call triggered a start of the QTimer
3. The timer itself was destroyed in the non-gui thread (in an active state), as was warned by Qt
4. The timer event was delivered to the destroyed timer, which caused a crash.

BUG:474476