external build: unregister javac extensions when build is finished

Otherwise if several builds are performed in the same java process (e.g. in tests) new instances of JavacFileReferencesRegistrar are registered in each 'buildStarted' method, and this causes failures if some instance become disabled in subsequent runs.

jps/jps-builders/src/org/jetbrains/jps/incremental/java/JavaBuilder.java

@@ -173,6 +173,7 @@ public class JavaBuilder extends ModuleLevelBuilder {
 
   @Override
   public void buildFinished(CompileContext context) {
+    myRefRegistrars.clear();
     final ConcurrentMap<String, Collection<String>> stats = COMPILER_USAGE_STATISTICS.get(context);
     if (stats.size() == 1) {
       final Map.Entry<String, Collection<String>> entry = stats.entrySet().iterator().next();