PrefService: fix bug in remove method

When removing a key, there are actually two different scenarios:

1. In the case of Map or List, the key is actually the name of a child
   node whose key/value pairs are the elements of the collection.
2. In the case of String or primitive values, the key really is a key
   in the backing preferences node.

In the case of (1), the child node should be removed, whereas in the
case of (2), the key should simply be removed. The code was handling
the former, but not the latter.

Author: ctrueden

src/main/java/org/scijava/prefs/DefaultPrefService.java

@@ -307,6 +307,7 @@ public SmartPrefs(final java.util.prefs.Preferences p,
 
 		public void remove(final String key) {
 			if (nodeExists(key)) node(key).removeNode();
+			p.remove(safeKey(key));
 		}
 
 		public void putMap(final Map<String, String> map) {