external build: don't try to load classes from jps-builders module in forked javac process

InProcessRefCollectorCompilerToolExtension is located in 'jps-builders' module, so if we switch jps-builders to Java 8 and try to compile a project using Javac 1.6 from forked process, it'll fail with UnsupportedClassVersionError.

jps/jps-builders-6/src/META-INF/services/org.jetbrains.jps.javac.JavaCompilerToolExtension

@@ -0,0 +1 @@
+org.jetbrains.jps.javac.ast.ExternalRefCollectorCompilerToolExtension

jps/jps-builders/src/META-INF/services/org.jetbrains.jps.javac.JavaCompilerToolExtension

@@ -1,2 +1 @@
-org.jetbrains.jps.javac.ast.ExternalRefCollectorCompilerToolExtension
 org.jetbrains.jps.javac.ast.InProcessRefCollectorCompilerToolExtension

jps/jps-builders/src/org/jetbrains/jps/cmdline/ClasspathBootstrap.java

@@ -31,7 +31,6 @@ import org.jetbrains.jps.builders.impl.java.EclipseCompilerTool;
 import org.jetbrains.jps.builders.java.JavaCompilingTool;
 import org.jetbrains.jps.builders.java.JavaSourceTransformer;
 import org.jetbrains.jps.javac.ExternalJavacProcess;
-import org.jetbrains.jps.javac.JavaCompilerToolExtension;
 import org.jetbrains.jps.javac.OptimizedFileManagerUtil;
 import org.jetbrains.jps.model.JpsModel;
 import org.jetbrains.jps.model.impl.JpsModelImpl;
@@ -39,9 +38,7 @@ import org.jetbrains.jps.model.serialization.JpsProjectLoader;
 import org.jetbrains.org.objectweb.asm.ClassVisitor;
 import org.jetbrains.org.objectweb.asm.ClassWriter;
 
-import javax.tools.JavaCompiler;
-import javax.tools.StandardJavaFileManager;
-import javax.tools.ToolProvider;
+import javax.tools.*;
 import java.io.File;
 import java.util.*;
 
@@ -166,10 +163,6 @@ public class ClasspathBootstrap {
       cp.add(getResourceFile(t.getClass()));
     }
 
-    for (JavaCompilerToolExtension toolExtension : JavaCompilerToolExtension.getExtensions()) {
-      cp.add(getResourceFile(toolExtension.getClass()));
-    }
-
     return new ArrayList<File>(cp);
   }