Remove gateway methods in Tramp

Remove gateway methods in Tramp

* doc/misc/tramp.texi (Top, Configuration): Remove section
`Gateway methods', insert section `Firewalls' in menu.
(History): Gateways are removed now.
(Gateway methods): Remove section.
(Multi-hops, Traces and Profiles): Don't reference to gateways anymore.
(Firewalls): New section.

* etc/NEWS: Gateway methods in Tramp have been removed.

* lisp/net/tramp.el (tramp-methods): Adapt docstring.
(tramp-file-name-port, tramp-accept-process-output): Simplify.

* lisp/net/tramp-gw.el: Remove.

* lisp/net/tramp-sh.el (tramp-gw-tunnel-method)
(tramp-gw-socks-method): Remove declarations.
(tramp-methods) <scp, scpx, ssh, sshx, telnet, nc, plink, pscp>:
Remove `tramp-gw-args' and `tramp-default-port'.  (Bug#18967)
(tramp-do-copy-or-rename-file-out-of-band)
(tramp-compute-multi-hops, tramp-maybe-open-connection):
Remove gateway support.

* test/lisp/net/tramp-tests.el (tramp-test03-file-name-defaults):
Remove gateway tests.