zeal: Use the main qt5 libraries set

Builds fine with it, so no need to pin an older version.

(This originally was triggered by qtwebengine failing for Qt514.
The build has been fixed on master in the meantime, but I guess it still
makes sense)

pkgs/top-level/all-packages.nix

···
     inherit (plasma5Packages) breeze-icons;
   };
 
-  zeal = libsForQt514.callPackage ../data/documentation/zeal { };
+  zeal = libsForQt5.callPackage ../data/documentation/zeal { };
 
   zilla-slab = callPackage ../data/fonts/zilla-slab { };