Replace custom unlink with BlockUnlinkAllCommand

Re-use existing unlink command and move all the unlinking code to
NodeRemoveCommand to cover all bases (deletion of a block in the
timeline or in the node editor).

app/widget/timelinewidget/timelinewidget.cpp

@@ -494,8 +494,6 @@ void TimelineWidget::ReplaceBlocksWithGaps(const QList<Block *> &blocks,
     new TrackReplaceBlockWithGapCommand(original_track, b, command);
 
     if (remove_from_graph) {
-      new BlockUnlinkAllCommand(b, command);
-
       new NodeRemoveWithExclusiveDeps(static_cast<NodeGraph*>(b->parent()), b, command);
     }
   }