[Fir2IR] Don't build overrides for expect classes.

We don't need them except for checking.
And checking doesn't work in any reasonable way anyway.

^KT-65249 Fixed

compiler/testData/codegen/box/multiplatform/k2/kt-65249.kt

@@ -0,0 +1,26 @@
+// LANGUAGE: +MultiPlatformProjects
+// MODULE: common
+// FILE: common.kt
+
+interface Source {
+    fun read(sink: Buffer): String
+}
+
+expect class Buffer()
+
+expect abstract class ForwardingSource: Source {
+    override fun read(sink: Buffer): String
+}
+
+// MODULE: jvm()()(common)
+// FILE: platfrom.kt
+
+actual class Buffer actual constructor()
+
+actual abstract class ForwardingSource : Source {
+    actual override fun read(sink: Buffer): String = "OK"
+}
+
+fun box() : String{
+    return (object : ForwardingSource() {}).read(Buffer())
+}