Remove support for rlogind in login(1), that is, remove the '-r' flag

The "quick hack" finally disappeared.  Probably nobody noticed.  ;)
(See the changes in <configure.ac> for the context of this pun.)

Probably everybody uses SSH these days for remote login.  Let's remove
this insecure method.

Closes: <https://github.com/shadow-maint/shadow/issues/992>
Reviewed-by: dkwo <nicolopiazzalunga@gmail.com>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Cc: "Serge E. Hallyn" <serge@hallyn.com>
Cc: Michael Vetter <jubalh@iodoru.org>
Cc: Sam James <sam@gentoo.org>
Cc: Benedikt Brinkmann <datacobra@thinkbot.de>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/prototypes.h

@@ -369,10 +369,6 @@ unsigned long csrand_interval (unsigned long min, unsigned long max);
 /* remove_tree.c */
 extern int remove_tree (const char *root, bool remove_root);
 
-/* rlogin.c */
-extern int do_rlogin(const char *remote_host, char *name, size_t namesize,
-                     char *term, size_t termsize);
-
 /* root_flag.c */
 extern void process_root_flag (const char* short_opt, int argc, char **argv);