boards: stm32f429i_disc1 remove usart2 node

On the STM32F429I-DISC1 PA2 is used for MEMS_INT2, which is driven low
by U3 by default. Connecting it to USART2 TX causes the MCU to try and
drive it high at the same time, causing excessive power consumption.

Dropping the usart2 node since the USART2 alternate pins are also used
for other things, and no other UARTs of the device has a pair of TX/RX
pins available on this reference board in its default configuration.

Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>