Remove impractical statement about backports

It has never been our policy to do backports for all intermediate
versions. It goes against a rapid release scheme: if we release every
few weeks, it's impractical to have to do backports against a
gazillion releases between the latest stable version used by NixOS and
the current Nix release.

doc/manual/src/release-notes/release-notes.md

@@ -5,8 +5,3 @@ Notable changes and additions are announced in the release notes for each versio
 
 Bugfixes can be backported on request to previous Nix releases.
 We typically backport only as far back as the Nix version used in the latest NixOS release, which is announced in the [NixOS release notes](https://nixos.org/manual/nixos/stable/release-notes.html#ch-release-notes).
-
-Backports never skip releases.
-If a feature is backported to version `x.y`, it must also be available in version `x.(y+1)`.
-This ensures that upgrading from an older version with backports is still safe and no backported functionality will go missing.
-