FIR: revert a hack that allows overriding T!! with T

1. this should've been only done if the language feature for validating
    that is disabled;

 2. that feature probably won't matter by the time FIR is stable;

 3. it only worked because type enhancement of type arguments is broken
    anyway - a more correct hack would be to provide a custom
    ConeTypePreparator.

compiler/testData/diagnostics/tests/explicitDefinitelyNotNullableViaIntersection/overridesJavaAnnotated.fir.kt

@@ -18,7 +18,7 @@ interface B<T1> : A<T1> {
 
 interface C<T2> : A<T2> {
     override fun foo(x: T2 & Any): T2 & Any
-    override fun bar(x: T2): <!RETURN_TYPE_MISMATCH_ON_OVERRIDE!>T2<!>
+    <!NOTHING_TO_OVERRIDE!>override<!> fun bar(x: T2): T2
 }
 
 interface D : A<String?> {