Remove historical comment in rust-toolchain.toml

A version of rustup which doesn't understand toml-formatted `rust-toolchain` files won't be looking in `rust-toolchain.toml`

The file was renamed to `rust-toolchain.toml` in #4204

rust-toolchain.toml support was added in rustup 1.23, which was released
on 2020-11-27 [1], 3 and a half years ago.

[1]: https://blog.rust-lang.org/2020/11/27/Rustup-1.23.0.html

Author: DJMcNab

rust-toolchain.toml

@@ -1,9 +1,3 @@
-# If you see this, run "rustup self update" to get rustup 1.23 or newer.
-
-# NOTE: above comment is for older `rustup` (before TOML support was added),
-# which will treat the first line as the toolchain name, and therefore show it
-# to the user in the error, instead of "error: invalid channel name '[toolchain]'".
-
 [toolchain]
 channel = "1.76"                     # Needed for deno & cts_runner. Firefox's MSRV is 1.74
 components = ["rustfmt", "clippy"]