Remove 'inner enum' hack & fix Java inner class resolve

Inner enum was placed into the class object of the outer class. Remove the
hack from frontend, frontend.java & backend. Fix tests

 #KT-1174 In Progress

compiler/testData/diagnostics/tests/enum/inner/insideInnerClassNotAllowed.kt

@@ -1,6 +1,6 @@
 class A {
-    class B {
-        <!ENUM_NOT_ALLOWED!>enum<!> class E {
+    inner class B {
+        enum class <!NESTED_CLASS_NOT_ALLOWED!>E<!> {
             ENTRY
         }
     }