Remove dependency on scala StringBuilder

Does not matter for JVM profiles but it's no longer used so no reason to
keep loading it.

Author: smarter

src/compiler/scala/tools/nsc/backend/jvm/BackendInterface.scala

@@ -708,7 +708,6 @@ abstract class BackendInterfaceDefinitions { self: BackendInterface =>
   val BoxedFloatClass: Symbol = requiredClass[java.lang.Float]
   val BoxedDoubleClass: Symbol = requiredClass[java.lang.Double]
   val StringClass: Symbol = requiredClass[java.lang.String]
-  val StringBuilderClass: Symbol = requiredClass[scala.collection.mutable.StringBuilder]
   val JavaStringBuilderClass: Symbol = requiredClass[java.lang.StringBuilder]
   val JavaStringBufferClass: Symbol = requiredClass[java.lang.StringBuffer]
   val JavaCharSequenceClass: Symbol = requiredClass[java.lang.CharSequence]