Make killing a non-last client work the same no matter the auto-stop setting

Make killing a non-last client work the same no matter the auto-stop setting

Previously, if 'server-stop-automatically' was configured for
'kill-terminal' or 'delete-frame', killing a client via
'save-buffers-kill-terminal' wouldn't prompt about the saving files in
the client's buffer list (as it does when not using those settings).
This change ensures that those settings only apply when killing the
last client, as described in the manual (bug#51993).

* lisp/server.el (server-save-buffers-kill-terminal): Handle
'server-stop-automatically' behavior in this function, rather than
calling 'server-stop-automatically--handle-delete-frame'.