gnu: gdk-pixbuf: Remove obsolete jasper support.

The newer meson 0.60 failed the build because the jasper flag no longer
exists.

* gnu/packages/gtk.scm (gdk-pixbuf)[configure-flags]<jasper>: Remove configure
flag.
[phases]{patch-docbook}: Remove condition and TODO comment.
[inputs]: Remove jasper.

gnu/packages/gtk.scm

@@ -639,15 +639,11 @@ highlighting and other features typical of a source code editor.")
     (outputs '("out" "debug"))
     (arguments
      `(#:glib-or-gtk? #t             ; To wrap binaries and/or compile schemas
-       #:configure-flags '("-Dinstalled_tests=false" "-Djasper=true")
+       #:configure-flags '("-Dinstalled_tests=false")
        #:phases
        (modify-phases %standard-phases
          (add-after 'unpack 'patch-docbook
-           ;; TODO(core-updates): Unconditionally look in (or native-inputs inputs)
-           (lambda* (#:key ,@(if (%current-target-system)
-                                 '(native-inputs)
-                                 '())
-                     inputs #:allow-other-keys)
+           (lambda* (#:key native-inputs inputs #:allow-other-keys)
              (with-directory-excursion "docs"
                (substitute* "meson.build"
                  (("http://docbook.sourceforge.net/release/xsl/current/")
@@ -684,7 +680,6 @@ highlighting and other features typical of a source code editor.")
      `(,@(if (%current-target-system)
              `(("bash-minimal" ,bash-minimal)) ; for glib-or-gtk-wrap
              '())
-       ("jasper" ,jasper)
        ("libjpeg" ,libjpeg-turbo)
        ("libpng"  ,libpng)
        ("libtiff" ,libtiff)))