Revert "WIP: Tweak handling of gnuplot-process"

This reverts commit ab859af9ddc6ed38b98b947bbac212778fff0aba.
Fix gnuplot-send-hiding-output

Use (get-buffer-process gnuplot-buffer) instead of gnuplot-process,
avoiding "Selecting deleted buffer" when gnuplot-process still refers to
an old process for some reason.
Get `gnuplot-process` via buffer, not name

Getting the process by name when setting up a new Gnuplot comint
in `gnuplot-make-gnuplot-buffer` is unreliable when the old process has
not yet been deleted, because Emacs will append `<n>` to the name of the
new process to make it unique.  Getting the process via the comint
buffer does not have this problem.
Cleanup `gnuplot-make-gnuplot-buffer`

- Use `unless` instead of `or`

- Check for process status of `gnuplot-process` being `run` instead of
  `(get-process gnuplot-process)`, which is a no-op AFAICT

- Use `buffer-live-p` instead of `buffer-name`

- Workaround `set-process-query-on-exit-flag` vs. obsolete
  `process-kill-without-query` functions