remove bindings codegen h/cpp files from .gitignore

These files don't exist in the tree, and the codegen doesn't create
them, so we shouldn't have them sitting around in .gitignore.
auto merge of #4238 : froydnj/servo/remove-cpp-files-from-gitignore, r=Manishearth

These files don't exist in the tree, and the codegen doesn't create them, so we shouldn't have them sitting around in .gitignore.

There are h/cpp files sitting around in the bindings codegen directory; I suppose one could make the argument that they're useful for examples, so we should keep them around, too?