boards: stm32: g474 nucleo board has no hw pins for usart2

Remove the USART 2 definition from the nucleo_g474re board
as it could cause a pin conflict on LPUART1 TX pin (PA2).
The USART2 is not present on this nucleo hardware and then
the g474re board definition is aligned with the nucleo_g431rb.

Signed-off-by: Francois Ramu <francois.ramu@st.com>