Remove support for {return,native,concrete}Type overrides in Bindings.conf.

We have no reason to support non-default type names, and this commit corrects
the computations for callbacks (which needed the override until now).
auto merge of #3632 : Ms2ger/servo/bindings-conf, r=Manishearth

We have no reason to support non-default type names, and this commit corrects
the computations for callbacks (which needed the override until now).