Remove myself as maintainer

[coriolinus]

@exercism/staff Please remove me as maintainer of the Rust track. I no longer have an appropriate amount of time to dedicate to this task. It's been good here!

[iHiD]

Thanks @coriolinus. I'll leave this for @ErikSchierboom to do when he's back from his break. Thanks for all your hard work on the Rust track - you've helped tens of thousands of people 💙

[ErikSchierboom]

I've removed you as a maintainer in our database. @iHiD will remove you as a maintainer on github. Thanks for all you've done!

[petertseng]

It's been a privilege.

As for how I see things proceeding from here, I'll hold down the fort on an emergency maintenance basis. Pull requests from contributors will be reviewed from time to time as my time allows, but it is unlikely that I will personally be able to make many significant improvements (e.g. concept exercises, following problem-specifications changes, test runner, etc.). At some point, other maintainers will need to fill the gaps.