Removed 'ensure' clause from session block

I've been encountering some real strangeness with using Birst_Command
in Rufus-scheduler and Rspec.  Sometimes when a Rufus-thread is terminated,
the 'logout' command is performed after the thread is terminated, which means
it runs in an unexpected place.  I couldn't resolve the issue other than removing the
ensure clause.  It's not worth fighting over anymore.  I don't expect this will cause
too many issues.  Logins should timeout by themselves.