Fix segfault on quit

Reproducible segmentation fault when start practice a collection and quit immediately from statistics main window without changing anything. The segfault is only shown on console output. DrKonqi window doesn't open. The segfault appears when ParleyDocument member is deleted in ParleyMainWindows destructor. The destroyed signal of ParleyDocuments KEduVocDocument member calls a slot to update the window cation which want to access an already deleted KEduVocDocument pointer. To fix this, the destroyed signal connection is removed before deleting the ParleyDocument member in ParleyMainWindows destructor.

BUG: 420302

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