Fix crash when removing Routes and VCA

VCATimeAxisView::self_delete () resets _vca and queues delete_when_idle.
From now on STAV::strippable () will return 0.

Editor::idle_remove_tracks() returns, and before the VCATimeAxisView
is destroyed. Editor::redisplay_track_views() may be called.

The VCATimeAxisView still exists at this point and is in
Editor::track_views, but has no valid stripable.

This causes a crash in TrackViewStripableSorter which depends
on STAV::stripable();

gtk2_ardour/vca_time_axis.cc

@@ -129,8 +129,6 @@ VCATimeAxisView::~VCATimeAxisView ()
 void
 VCATimeAxisView::self_delete ()
 {
-	/* reset reference immediately rather than deferring to idle */
-	_vca.reset ();
 	delete_when_idle (this);
 }