MAINTAINERS: Remove Richard Purdie from LED maintainers

Richard has been inactive on the linux-leds list for a long time.
After email discussion we agreed on removing him from
the LED maintainers, which will better reflect the actual status.

Acked-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>