ramus, tangra, flake, overlays: drop php backport (nixpkgs#576853 landed)

nixos-unstable 3ec6363 ships php83 8.3.36 / php85 8.5.12; the overlay
was warning that it had become a no-op.

machines/ramus/think-greater-chiangmai.nix

@@ -56,7 +56,6 @@ in
   ];
   nixpkgs.overlays = [
     self.overlays.packagesOverlay
-    self.overlays.phpPr576853Overlay # TEMPORARY, see overlays/nixpkgs-pr-576853-php.nix
   ];
 
   clan.core.vars.generators.greaterchiangmai = {