Drop KotlinTarget.INNER_CLASS entry

In most of the cases it was used together with CLASS_ONLY and vice versa
They only case when CLASS_ONLY was used without INNER_CLASS
is possibleParentTargetMap.COMPANION_KEYWORD.

But after diagnostic NESTED_OBJECT_NOT_ALLOWED has been introduced,
there's no sense in the restriction of exactly the companion's parent

For clarification see test data changed

compiler/testData/diagnostics/tests/classObjects/InnerClassClassObject.kt

@@ -2,7 +2,7 @@
 
 class A {
     inner class B {
-        <!WRONG_MODIFIER_CONTAINING_DECLARATION!>companion<!> object { }
+        companion <!NESTED_OBJECT_NOT_ALLOWED!>object<!> { }
     }
 }
 
@@ -22,4 +22,4 @@ class C {
     class D {
         companion object { }
     }
-}
+}