A test gap in `CheckInvocationArgMixing` method

[AlekseyTs]

            if (receiverType?.IsRefLikeOrAllowsRefLikeType() == true && !IsReceiverRefReadOnly(symbol))
            {
                // PROTOTYPE(RefStructInterfaces): We do not have a test that demonstrates that the statement below makes a difference.
                //                                 If it is commented out, not a single test fails 
                escapeTo = GetValEscape(receiverOpt, scopeOfTheContainingExpression);
            }

[jaredpar]

Good catch. This code path is unnecessary. Sending a fix to delete and explanation of why.