cirrus-ci: Drop obsolete dependency

markdown as a dependency was dropped in 4.12

Fixes: 5d94433a66 ("cirrus-ci: introduce some basic FreeBSD testing")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>

.cirrus.yml

@@ -4,7 +4,7 @@ freebsd_template: &FREEBSD_TEMPLATE
     APPEND_LIB: /usr/local/lib
     APPEND_INCLUDES: /usr/local/include
 
-  install_script: pkg install -y seabios markdown gettext-tools gmake
+  install_script: pkg install -y seabios gettext-tools gmake
                                  pkgconf python libiconv bison perl5
                                  yajl lzo2 pixman argp-standalone
                                  libxml2 glib git