firefox: drop hint regarding system cairo

Building with system cairo has been unsupported since Firefox 60
(mozbz#1432751) and the flag was removed in 2021 with Firefox 92
(mozbz#1722653).

pkgs/applications/networking/browsers/firefox/common.nix

@@ -260,7 +260,6 @@ buildStdenv.mkDerivation ({
     "--with-system-icu"
     "--enable-system-ffi"
     "--enable-system-pixman"
-    #"--enable-system-cairo"
     "--disable-tests"
     "--disable-updater"
     "--enable-default-toolkit=${default-toolkit}"