Merge pull request #235459 from agbrooks/remove-defunct-mirror

fetchurl: remove defunct mirror kent.dl.sourceforge.net

pkgs/build-support/fetchurl/mirrors.nix

···
     "https://versaweb.dl.sourceforge.net/sourceforge/"
     "https://freefr.dl.sourceforge.net/sourceforge/"
     "https://osdn.dl.sourceforge.net/sourceforge/"
-    "https://kent.dl.sourceforge.net/sourceforge/"
   ];
 
   # Steam Runtime