This avoids having a dummy event to tickle the main loop.
Confirmed using `nvim -u NONE -c 'au FocusGained * q'` in tmux (with
`:set -g focus-events on`): without the flushing it would only exit
after pressing a key.
Moves the flushing done recently in 3626d2107.
`nvim -u NONE -cq` is still working (i.e. consuming the response for the
terminal background query itself), and the flickering mentioned in
3626d2107 is reduced again.
Reverts part of bfb21f3e0 (#7729).