Rework tmux dev setup spawning

Remove the unrequired -2 option when attaching a tmux session. Get rid
of the warning when selecting the window by providing the missing + and
remove the unrequired split-window.
3 jobs for master in 26 seconds (queued for 1 second)
Status Name Job ID Coverage
  Test
passed dist #9870

00:00:24

passed gemnasium-dependency_scanning #9868

00:00:15

passed retire-js-dependency_scanning #9869

00:00:16