serial: Remove duplicated line in Makefile

The line "-obj-$(CONFIG_STM32_SERIAL) += serial_stm32.o"
is found twice in Makefile.

Fixes: ae74de0d ("serial: stm32: Rename serial_stm32x7.c to serial_stm32.c"

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Vikas Manocha <vikas.manocha@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>