Remove the SourceGroup workaround when source are deleled...

[NicoKiaru]

... as soon as bigdataviewer/bigdataviewer-core#177 is fixed

[NicoKiaru]

This mess:

bigdataviewer-selector/src/main/java/ch/epfl/biop/bdv/select/SourceSelectorBehaviour.java

Lines 183 to 188 in ebd141a

	bdvh.getViewerPanel().state().getGroups().forEach(
	group -> {
	bdvh.getViewerPanel().state().addSourcesToGroup(Collections.singleton(getSourceAndConverterFrom(bos)), group);
	sourceGroups.add(group); // To remind us to remove the overlay when it's uninstalled
	}
	);

[NicoKiaru]

See also bigdataviewer/bigdataviewer-playground#272 and close it, as soon as the timepoint behaviour is checked.