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/multipleDissallowedDefaultObjects.kt

@@ -1,10 +1,10 @@
 class A {
     inner class I {
-        <!WRONG_MODIFIER_CONTAINING_DECLARATION!>companion<!> object A
+        companion <!NESTED_OBJECT_NOT_ALLOWED!>object A<!>
 
-        <!MANY_COMPANION_OBJECTS, WRONG_MODIFIER_CONTAINING_DECLARATION!>companion<!> object B
+        <!MANY_COMPANION_OBJECTS!>companion<!> <!NESTED_OBJECT_NOT_ALLOWED!>object B<!>
 
-        <!MANY_COMPANION_OBJECTS, WRONG_MODIFIER_CONTAINING_DECLARATION!>companion<!> object C
+        <!MANY_COMPANION_OBJECTS!>companion<!> <!NESTED_OBJECT_NOT_ALLOWED!>object C<!>
     }
 }
 
@@ -14,4 +14,4 @@ object O {
     <!MANY_COMPANION_OBJECTS, WRONG_MODIFIER_CONTAINING_DECLARATION!>companion<!> object B
 
     <!MANY_COMPANION_OBJECTS, WRONG_MODIFIER_CONTAINING_DECLARATION!>companion<!> object C
-}
+}