gnu: Remove Krita 3.

Krita 3 fails to build with Qt 5.11.1. Since it's also unmaintained upstream,
remove it.

* gnu/packages/kde.scm (krita-3): Remove variable.

gnu/packages/kde.scm

@@ -329,23 +329,6 @@ illustrators, matte and texture artists, and the VFX industry.  Notable
 features include brush stabilizers, brush engines and wrap-around mode.")
     (license license:gpl2+)))
 
-;; Krita 3 and 4's file formats are incompatible, so we are keeping Krita 3
-;; for now.
-(define-public krita-3
-  (package
-    (inherit krita)
-    (name "krita")
-    (version "3.3.3")
-    (source (origin
-              (inherit (package-source krita))
-              (uri (string-append
-                    "mirror://kde/stable/krita/"
-                    (version-prefix version 3)
-                    "/" name "-" version ".tar.gz"))
-              (sha256
-               (base32
-                "0pc6hnakkqy81x5b5ncivaps6hqv43i50sjwgi3i3cz9j8rlxh5y"))))))
-
 (define-public kholidays
   (package
     (name "kholidays")