services: Remove deprecated service procedures.
These service procedures were deprecated in January 2019, for instance
in commit 65a67bf711.
* gnu/services/avahi.scm (avahi-service): Remove.
* gnu/services/base.scm (console-keymap, guix-service)
(guix-publish-service, gpm-service, urandom-seed-service): Remove.
* gnu/services/desktop.scm (upower-service, colord-service): Remove.
* gnu/services/mcron.scm (mcron-service): Remove.
* gnu/services/messaging.scm (bitlbee-service): Remove.
* gnu/services/networking.scm (dhcp-client-service, ntp-service)
(tor-service): Remove.
* gnu/services/xorg.scm (slim-service, gdm-service): Remove.
f70c3429ed