No override check in FirClassUseSiteScope, fix nasty substitution bug

This fixes MPP override test
(see mppFakeOverride in FirMultiModuleResolveTestGenerated)

idea/testData/fir/multiModule/overrideWithJava/jvm/B.kt

@@ -1,6 +1,6 @@
 class B : A() {
     override fun foo(): B = this
-    fun bar(): B = this // Ambiguity, no override here
+    fun bar(): B = this // Here we should have "missing override" but no ambiguity
 
     fun test() {
         foo()