Delete all SLUGS-autopilot-specific code

This code has been unused for a few years at this point with that project currently handling maintaining their own fork. The eventual plan is to merge their changes back in later this year. This merge will be easier to review if it's a clean addition of code versus a heavy modification of this now old & dead code.

Compilation succeeds with the Ardupilot and the SLUGS autopilot selected in my `user_config.pri` file.

@dagoodma is that project lead currently, so pinging him on this.