Revert commit because two commits fixing separate bugs were accidentally stuck together. Will recommit bugfixes separately.

Revert "KT-2193 Nullability information lost for functions inherited from traits".

This reverts commit d60818fd3e.

compiler/testData/readKotlinBinaryClass/classFun/FunDelegationToTraitImpl.txt

@@ -1,9 +0,0 @@
-namespace test
-
-abstract trait test.A : jet.Any {
-    open fun f(): jet.String
-}
-final class test.B : test.A {
-    final /*constructor*/ fun <init>(): test.B
-    open override /*1*/ fun f(): jet.String
-}