imagemagick: remove mirrors that don't work

/cc #8302.

pkgs/build-support/fetchurl/mirrors.nix

···
 
   # ImageMagick mirrors, see http://www.imagemagick.org/script/download.php.
   imagemagick = [
-    ftp://ftp.nluug.nl/pub/ImageMagick/
+    http://www.imagemagick.org/download/
+    ftp://ftp.sunet.se/pub/multimedia/graphics/ImageMagick/ # also contains older versions removed from most mirrors
     ftp://ftp.imagemagick.org/pub/ImageMagick/
-    ftp://ftp.imagemagick.net/pub/ImageMagick/
-    ftp://ftp.sunet.se/pub/multimedia/graphics/ImageMagick/
   ];
 
   # CPAN mirrors.