smoketest: T6539: remove explicit test for non existing global log entry

Commit dd5908eac ("T6539: add logging options to load-balancer reverse-proxy")
added CLI commands to add custom log entries for frontend and backend servers
as well as a global log directive.

Remove explicit test code for no longer always present "global log" directive.

(cherry picked from commit b71481f7de93d43ff7208ff76cbc6b829b8951b2)