marst,libidn2,mdk: remove duplicate /gnu in the url

there urls only worked with "https://ftpmirror.gnu.org/" because it
ignored the extra /gnu

while with the other gnu mirrors "https://ftp.nluug.nl/pub/gnu/" it
failed because the url would be pub/gnu/gnu.

pkgs/development/compilers/marst/default.nix

···
   version = "2.7";
 
   src = fetchurl {
-    url = "mirror://gnu/gnu/${pname}/${pname}-${version}.tar.gz";
+    url = "mirror://gnu/${pname}/${pname}-${version}.tar.gz";
     hash = "sha256-Pue50cvjzZ+19iJxfae7VQbxpto7MPgS4jhLh7zk2lA=";
   };
 

pkgs/development/libraries/libidn2/default.nix

···
   version = "2.3.2";
 
   src = fetchurl {
-    url = "mirror://gnu/gnu/libidn/${pname}-${version}.tar.gz";
+    url = "mirror://gnu/libidn/${pname}-${version}.tar.gz";
     sha256 = "sha256-dpQM1Od46Ak1eanRlbJf/16Tbp3GJCBoUotDenZ2T5E=";
   };
 

pkgs/development/tools/mdk/default.nix

···
   pname = "gnu-mdk";
   version = "1.3.0";
   src = fetchurl {
-    url = "mirror://gnu/gnu/mdk/v${version}/mdk-${version}.tar.gz";
+    url = "mirror://gnu/mdk/v${version}/mdk-${version}.tar.gz";
     sha256 = "0bhk3c82kyp8167h71vdpbcr852h5blpnwggcswqqwvvykbms7lb";
   };
   nativeBuildInputs = [ pkg-config intltool ];