Remove reference to Gitter (#1571)

The Exerism Forum (https://forum.exercism.org/) has now made Gitter redundant.
We're directing everyone there.

Author: kytrinyx

README.md

@@ -1,7 +1,6 @@
 # Exercism Rust Track
 
 [![CI](https://github.com/exercism/rust/workflows/CI/badge.svg?branch=main)](https://github.com/exercism/rust/actions?query=workflow%3ACI+branch%3Amain)
-[![Join the chat at https://gitter.im/exercism/rust](https://badges.gitter.im/exercism/rust.svg)](https://gitter.im/exercism/rust?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 Exercism exercises in Rust