Don't generate generic signature for get/set methods in property reference class

compiler/testData/writeSignature/callableFunction.kt

@@ -0,0 +1,15 @@
+class Foo() {
+    fun test(): String = "OK"
+}
+
+fun test(s: () -> String): String {
+    return s()
+}
+
+fun box(): String {
+    return test(Foo()::test)
+}
+
+// method: CallableFunctionKt$box$1::invoke
+// jvm signature:     ()Ljava/lang/String;
+// generic signature: null