Cleaning up old C header code from convert, we'll just deal with Khronos
XML files now. If there's a need for parsing C headers we should do that
as a separate tool that either parses and generates the OpenTK XML specs
directly generates Khronos XML specs that can then be parsed by Convert.
Khronos stopped supporting .spec files in 2013. They're now over a year
old without support. As such it's not worth us continuing to support
them either.