boards: stm32f769i_disco: drop led_4 node

The LED 4 is connected to the STMPS2151STR on the FAULT output pin which
is pulled low, automatically turning on the LED, in case of current
limit faults. Therefore, pin D4 should not be configured as a LED but
rather as an input to detect the fault condition.

Additionally, it should be noted that the board's silkscreen labels this
LED as "OC" (OverCurrent) rather than usr{1,2,3}, as is the case for the
other LEDs described in the DTS.

Fixes: cc5ae491
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>