HID: logitech: drop outdated references to unifying receivers

The hid-logitech-{dj,hidpp} were originally developed for unifying
receivers but since then they have evolved and now support other types
of receivers and devices. This patch adjusts the original descriptions
with this in mind.

Signed-off-by: Filipe Laíns <lains@archlinux.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>