Documentation: leds: remove invalidated information

The contents of the Future Development section of leds-class
Documentation was invalidated when support for LED-private triggers
was merged. Remove this section.

Signed-off-by: Marek Behún <kabel@kernel.org>
Fixes: 93690cdf ("leds: trigger: add support for LED-private device...")
Signed-off-by: Pavel Machek <pavel@ucw.cz>