fetchurl: Always use tarballs.nixos.org

Otherwise, if the upstream mirror changes (rather than deletes) a
file, then tarballs.nixos.org won't be used even if it has a copy of
the original file, and so we'll get a hash mismatch.

pkgs/build-support/fetchurl/builder.sh

@@ -89,10 +89,6 @@ for url in $urls; do
         if test -z "${!varName}"; then
             echo "warning: unknown mirror:// site \`$site'"
         else
-            # Assume that SourceForge/GNU/kernel mirrors have better
-            # bandwidth than nixos.org.
-            preferHashedMirrors=
-
             mirrors=${!varName}
 
             # Allow command-line override by setting NIX_MIRRORS_$site.