Speed up LXD precache

Speed up the caching of LXD images used by the installer and daily cron
by removing the trusty image, which is no longer used by default.

Signed-off-by: Adam Israel <adam.israel@canonical.com>
diff --git a/jenkins/VCA/update-lxd-image.sh b/jenkins/VCA/update-lxd-image.sh
index 81b210c..8cad93b 100755
--- a/jenkins/VCA/update-lxd-image.sh
+++ b/jenkins/VCA/update-lxd-image.sh
@@ -51,5 +51,6 @@
     lxc delete $container -f || true
 }
 
-cache trusty "$TRUSTY_PACKAGES"
+# Cache the image for the Ubuntu series or series to support
+# cache trusty "$TRUSTY_PACKAGES"
 cache xenial "$XENIAL_PACKAGES"