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

compiler/testData/writeSignature/callableProperty.kt

@@ -0,0 +1,13 @@
+class Foo(val a: String)
+
+fun test(s: () -> String): String {
+    return s()
+}
+
+fun box(): String {
+    return test(Foo("OK")::a)
+}
+
+// method: CallablePropertyKt$box$1::get
+// jvm signature:     ()Ljava/lang/Object;
+// generic signature: null