Remove jextract dependency

[jcrist1]

In order to be more tightly integrated with the rest of diplomat (and merged in tree) we would like to remove the dependency on jextract, and generate the JEP 454 binding code directly. This should be relatively straightforward as we can build on the existing jextract code, but only generate code that we actually use in diplomat bindings. Jextract also seems to generate code that we wouldn't strictly need.