Added test for obsolete bug and removed workaround in code.

compiler/testData/diagnostics/tests/overload/kt2493.kt

@@ -0,0 +1,19 @@
+trait A
+trait B
+
+fun <R: A> R.f() {
+}
+
+fun <R: B> R.f() {
+}
+
+class AImpl: A
+class BImpl: B
+
+class C: A, B
+
+fun main(args: Array<String>) {
+    AImpl().f()
+    BImpl().f()
+    C().<!CANNOT_COMPLETE_RESOLVE!>f<!>()
+}