docs: rust: quick-start: remove Nix "unstable channel" note

docs: rust: quick-start: remove Nix "unstable channel" note

Nix does not need the "unstable channel" note, since its packages are
recent enough even in the stable channel [1][2].

Thus remove it to simplify the documentation.

Link: https://search.nixos.org/packages?channel=25.11&query=rust
Link: https://search.nixos.org/packages?channel=25.11&query=bindgen
Reviewed-by: Tamir Duberstein <tamird@kernel.org>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260405235309.418950-28-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>