leds: leds-fsg: Drop FSG3 LED driver

The board file using this driver has been deleted and the
FSG3 LEDs can be modeled using a system controller and some
register bit LEDs in the device tree so this driver is no
longer needed.

Reported-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Krzysztof Hałasa <khalasa@piap.pl>
Cc: Rod Whitby <rod@whitby.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>