cirrus-ci: Remove flex/bison packages

QEMU does not use flex/bison packages.

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Li-Wen Hsu <lwhsu@freebsd.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200515163029.12917-5-philmd@redhat.com>

.cirrus.yml

@@ -7,7 +7,7 @@ freebsd_12_task:
     cpu: 8
     memory: 8G
   install_script: ASSUME_ALWAYS_YES=yes pkg bootstrap -f ; pkg install -y
-    bash bison curl cyrus-sasl git glib gmake gnutls gsed
+    bash curl cyrus-sasl git glib gmake gnutls gsed
     nettle perl5 pixman pkgconf png usbredir
   script:
     - mkdir build