gnu: gtk-doc: Remove leftovers.

No longer needed, package passes tests without these parameters.

* gnu/packages/gtk.scm (gtk-doc)[arguments]<#:parallel-tests?>: Re-enable.
<#:phases>: Remove 'disable-failing-tests.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>

gnu/packages/gtk.scm

@@ -2260,14 +2260,8 @@ information.")
     (build-system meson-build-system)
     (arguments
      (list
-      #:parallel-tests? #f
       #:phases
       #~(modify-phases %standard-phases
-         (add-after 'unpack 'disable-failing-tests
-           (lambda _
-             (substitute* "tests/Makefile.am"
-               (("annotations.sh bugs.sh empty.sh fail.sh gobject.sh program.sh")
-                ""))))
          (add-after 'install 'wrap-executables
            (lambda _
              (let ((docbook-xsl-catalog