icu: keep default version on 58

... to avoid mass rebuilds for now. (Bumped in parent merge.)

pkgs/top-level/all-packages.nix

···
   icu58 = callPackage ../development/libraries/icu/58.nix { };
   icu59 = callPackage ../development/libraries/icu/59.nix { };
 
-  icu = icu59;
+  icu = icu58;
 
   id3lib = callPackage ../development/libraries/id3lib { };