Don't explicitly delete client frames when killing Emacs anyway

Don't explicitly delete client frames when killing Emacs anyway

This eliminates a useless error prompt when killing Emacs from a
client frame when there are no other frames (bug#58877).

* lisp/server.el (server-running-external): New error.
(server--file-name): New function...
(server-eval-at): ... use it.
(server-start): Factor out server stopping code into...
(server-stop): ... here.
(server-force-stop): Use 'server-stop', and tell it not to delete
frames.

* test/lisp/server-tests.el
(server-tests/server-force-stop/keeps-frames): New test.