Replace hasInterfacePrototypeObject checks by isCallback checks in codegen.

Checking for callbacks directly makes it a lot clearer what's going on, and we
don't intend to take hasConcreteDescendant into consideration.

See also <https://bugzilla.mozilla.org/show_bug.cgi?id=1026720>.
Merge pull request #3112 from Ms2ger/hasInterfacePrototypeObject

Replace hasInterfacePrototypeObject checks by isCallback checks in codegen.