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/inner/nestedObject.kt

@@ -3,7 +3,7 @@ class Outer {
     inner class Inner1 {
         <!NESTED_OBJECT_NOT_ALLOWED!>object Obj1<!>
 
-        <!WRONG_MODIFIER_CONTAINING_DECLARATION!>companion<!> object Obj2
+        companion <!NESTED_OBJECT_NOT_ALLOWED!>object Obj2<!>
 
         inner class Inner2 {
             <!NESTED_OBJECT_NOT_ALLOWED!>object Obj3<!>