Remove Java implementation of the gRPC server

This makes 'make java' go from 11 to 8 seconds (with a warm cache of dependencies), and the uber JAR from 60 to 46MB.

(Marking as draft, because most of the Java code is only tested through the gRPC server, and I want to keep those tests in case I need to make a change to the rest of the Java code before I delete it too)

Closes #4763 (closed) and #4768 (closed)