Make sanity check in CanvasSwitcher less strict

I couldn't reproduce the crash, but the mechanics happens like that:

1. KisCanvas2::setCanvasWidget() adds the canvas to the list of the tracked ones and continues loading process.

2. Some other canvas gets activated by spontaneous SetFocus event (Note: I haven't managed to reproduce that!)

3. KisMainWindow::addView() registers the canvas at the input manager again. But since some other canvas has been activated by a spontaneous event, d->canvas is not equal to canvas anymore.

Possible solutions:

Option A:
Remove double registration of the canvas in the input manager and do
it only once either in KisCanvas::setCanvasWidget() or in KisMainWindow

Option B:
Justremove the sanity check. But this way we may miss real bugs that
may happen because of double initialization of the canvas.

CCBUG:421518