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/twoEnumsInClassObjectAndInnerClass.kt

@@ -3,7 +3,7 @@ class A {
         enum class E { ENTRY }  // OK
     }
     
-    class B {
-        <!ENUM_NOT_ALLOWED!>enum<!> class E { ENTRY }
+    inner class B {
+        enum class <!NESTED_CLASS_NOT_ALLOWED!>E<!> { ENTRY }
     }
 }