Remove getDefaultRetval.

getDefaultRetval was supposed to be used after throwing an exception on the
ErrorResult outparam in C++. However, as we throw exceptions through the
return value in Servo, we don't need it at all.
Stop returning a default return value from getRetvalInfo.

This is now unused.
auto merge of #2452 : Ms2ger/servo/getDefaultRetval, r=jdm