dts: stm32f4-pinctrl remove wrong pinmux configurations

Remove usart2_pins_b as this configuration is not possible
(PA15 could not be mapped on USART2).
Besides usart2_pins_c as this configuration is not used yet,
remove to reserve "usart2_pins_c" for future use

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>