Remove some code to support preffing off APIs.

We won't be using this feature in the near future, and the implementation is
not really Rustic.
Pass slices to CreateInterfaceObjects2 rather than raw pointers.
Remove the pointless terminator from ConstantSpec slices.
Store a slice in ConstantSpec, rather than a raw pointer.
auto merge of #2599 : Ms2ger/servo/constantspec, r=jdm