HID: thingm: remove the "play" sysfs attribute

When the thingm driver registers an instance of LED class, it creates a
"play" sysfs attribute for this blink(1) specific feature.

Since this feature is not specific to the RGB chip but to the HID device
itself, let's remove this attribute from the LED instance and only
implement what is useful to switch on and off the LED.

This feature is still easily accessible through hidraw.

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