Fix simplify_return function when passed the 'abstype'

I have encountered an error.

$ ./erlang_complete.erl array
escript: exception error: {function_clause,
                     [{local,simplify_return,
                          [{abstype,
                               [{href,"#type-array"}],
                               [{erlangName,[{name,"array"}],[]},
                                {type,
                                    [{name,"Type"}],
                                    [{typevar,[{name,"Type"}],[]}]}]}]}]}