MAINTAINERS: remove include/linux/of_pwm.h

Added by commit 200efedd ("pwm: Take over maintainership of the PWM
subsystem"), but I could not find any trace of that file being ever
added to the repository.

Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Acked-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>