chore(rust): drop rustfmt from rust-toolchain.toml

This just installs regular rustfmt, which is not needed in this project.
One could say "It doesn't hurt", but in the NixOS dev shell it actually
does since it will shadow nightly rustfmt and we don't have the
`cargo +nightly fmt` synatx on NixOS that is available on other Distros.

Also "It doesn't hurt" to delete it for non NixOS users.

rust-toolchain.toml

@@ -16,6 +16,9 @@ components = [
     "rust-src",
     "rust-analyzer",
     # For CI and editors
-    "rustfmt",
     "clippy",
+    # you have to install rustfmt nightly yourself (if you're not on NixOS)
+    #
+    # The rust-toolchain.toml file doesn't provide any syntax for specifying components from different toolchains
+    # "rustfmt"
 ]