Kill pane 0 when ssm-session tmux started without attaching

[rothgar]

If you're in a nested $TMUX session and run ssm-session with multiple instances a new tmux session will be created called ssm-session by default and not attached.

By default the new session does not have pane 0 killed because of the way we automate the pane creation in gomux. Might be a way to fix that so there is not an extra pane in the new, detached session.