HID: thingm: remove the "fade" sysfs attribute

As for the "play" sysfs attribute, remove this other non-standard
attribute, so the driver only implements what is required to switch the
LED on and off. Thus, a fade time won't be ideal for some fast-changing
triggers.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>