MAINTAINERS: remove Richard Purdie as backlight maintainer

Remove Richard Purdie as backlight subsystem maintainer,

akpm: Richard is still responsive and is reviewing some of the patches,
but appears to agree that listing him as the maintainer is no longer
appropriate.

[akpm@linux-foundation.org: s/USA/UK/]
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>