Revert "plasma5: mark broken with Qt >= 5.15"

This reverts commit 575b3a053a.

We don't know why ttuegel did it, and we suspect it was due to packages
breaking due to mixed qt5 versions used together, and not plasma itself
with qt5.15 - (#101369).

pkgs/desktops/plasma-5/default.nix

@@ -36,7 +36,6 @@ let
   };
 
   mkDerivation = libsForQt5.callPackage ({ mkDerivation }: mkDerivation) {};
-  qtbase = libsForQt5.callPackage ({ qtbase }: qtbase) {};
 
   packages = self: with self;
     let
@@ -83,7 +82,6 @@ let
             setupHook = args.setupHook or defaultSetupHook;
 
             meta = {
-              broken = lib.versionAtLeast qtbase.version "5.15";
               license = with lib.licenses; [
                 lgpl21Plus lgpl3Plus bsd2 mit gpl2Plus gpl3Plus fdl12
               ];