Simplifies the check for installed nginx, fixes purge when nginx was not used.

debian/jitsi-meet.postinst

         # stores the hostname so we will reuse it later, like in purge
         db_set jitsi-meet/jvb-hostname $JVB_HOSTNAME
 
-        NGINX_INSTALL_CHECK="$(dpkg-query -W -f '${PackageSpec}:${Status}\n' nginx 2>&1 | grep -v "ok installed" || :)"
-        if [ -z "${NGINX_INSTALL_CHECK}" ]; then
+        NGINX_INSTALL_CHECK="$(dpkg-query -f '${Status}' -W 'nginx' 2>/dev/null | awk '{print $3}' || true)"
+        if [ "$NGINX_INSTALL_CHECK" = "installed" ] || [ "$NGINX_INSTALL_CHECK" = "unpacked" ] ; then
             FORCE_NGINX="true"
         fi
 

debian/jitsi-meet.postrm

             rm -f /etc/jitsi/meet/$JVB_HOSTNAME-config.js
             rm -f /etc/nginx/sites-available/$JVB_HOSTNAME.conf
             rm -f /etc/nginx/sites-enabled/$JVB_HOSTNAME.conf
+            rm -f /etc/jitsi/videobridge/$JVB_HOSTNAME.jks
+            rm -f /etc/jitsi/videobridge/$JVB_HOSTNAME.p12
         fi
     ;;
     upgrade|failed-upgrade|abort-install|abort-upgrade|disappear)