Remove nonsensical body for trait getter

This corrects an error in the change to the trait encoding
in scala#5003: getters in traits should have empty bodies and
be emitted as abstract.

```
% ~/scala/2.12.0-M4/bin/scalac sandbox/test.scala && javap -c T
Compiled from "test.scala"
public interface T {
  public abstract void T$_setter_$x_$eq(int);

  public int x();
    Code:
       0: aload_0
       1: invokeinterface #15,  1           // InterfaceMethod x:()I
       6: ireturn

  public int y();
    Code:
       0: aload_0
       1: invokeinterface #20,  1           // InterfaceMethod y:()I
       6: ireturn

  public void y_$eq(int);
    Code:
       0: aload_0
       1: iload_1
       2: invokeinterface #24,  2           // InterfaceMethod y_$eq:(I)V
       7: return

  public void $init$();
    Code:
       0: aload_0
       1: bipush        42
       3: invokeinterface #29,  2           // InterfaceMethod T$_setter_$x_$eq:(I)V
       8: aload_0
       9: bipush        24
      11: invokeinterface #24,  2           // InterfaceMethod y_$eq:(I)V
      16: return
}

% qscalac sandbox/test.scala && javap -c T
Compiled from "test.scala"
public interface T {
  public abstract void T$_setter_$x_$eq(int);

  public abstract int x();

  public abstract int y();

  public abstract void y_$eq(int);

  public static void $init$(T);
    Code:
       0: aload_0
       1: bipush        42
       3: invokeinterface #21,  2           // InterfaceMethod T$_setter_$x_$eq:(I)V
       8: aload_0
       9: bipush        24
      11: invokeinterface #23,  2           // InterfaceMethod y_$eq:(I)V
      16: return

  public void $init$();
    Code:
       0: aload_0
       1: invokestatic  #27                 // Method $init$:(LT;)V
       4: return
}
```

Author: retronym

src/compiler/scala/tools/nsc/transform/Mixin.scala

@@ -1001,8 +1001,13 @@ abstract class Mixin extends InfoTransform with ast.TreeDSL {
           val parents1 = currentOwner.info.parents map (t => TypeTree(t) setPos tree.pos)
           // mark fields which can be nulled afterward
           lazyValNullables = nullableFields(templ) withDefaultValue Set()
+          val bodyEmptyAccessors = if (!sym.enclClass.isTrait) body else body mapConserve {
+            case dd: DefDef if dd.symbol.isAccessor && !dd.symbol.isLazy =>
+              deriveDefDef(dd)(_ => EmptyTree)
+            case tree => tree
+          }
           // add all new definitions to current class or interface
-          treeCopy.Template(tree, parents1, self, addNewDefs(currentOwner, body))
+          treeCopy.Template(tree, parents1, self, addNewDefs(currentOwner, bodyEmptyAccessors))
 
         case Select(qual, name) if sym.owner.isTrait && !sym.isMethod =>
           // refer to fields in some trait an abstract getter in the interface.