speakersafetyd: drop User=speakersafetyd line

The NixOS module, living in nix-community/nixos-apple-silicon, doesn't
set up this user, and it's ok for now to just keep running as root.

Long-term, speakersafetyd should support running via DynamicUser=yes,
see the discussion in https://github.com/AsahiLinux/speakersafetyd/pull/26.

pkgs/by-name/sp/speakersafetyd/package.nix

@@ -34,6 +34,8 @@ rustPlatform.buildRustPackage (finalAttrs: {
 
   postPatch = ''
     substituteInPlace speakersafetyd.service \
+      --replace-fail "User=speakersafetyd" \
+                     "" \
       --replace-fail "/usr" \
                      "$out"