Remove assertion

Apparently this is reachable under normal circonstances
(tests/pos-deep-subtype/i5876.scala), but throwing a CyclicReference
execption sounds like the currect thing to do here, right?

Author: OlivierBlanvillain

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -2876,11 +2876,6 @@ object Types {
     def ref(using Context): Type =
       if computed then
         if myRef == null then
-          // if errors were reported previously handle this by throwing a CyclicReference
-          // instead of crashing immediately. A test case is neg/i6057.scala.
-          assert(ctx.mode.is(Mode.CheckCyclic)
-              || ctx.mode.is(Mode.Printing)
-              || ctx.reporter.errorsReported)
           throw CyclicReference(NoDenotation)
       else
         computed = true