serial: Remove duplicated line in Makefile

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

Fixes: ae74de0dfd ("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>

drivers/serial/Makefile

@@ -60,7 +60,6 @@ obj-$(CONFIG_UNIPHIER_SERIAL) += serial_uniphier.o
 obj-$(CONFIG_STM32_SERIAL) += serial_stm32.o
 obj-$(CONFIG_STI_ASC_SERIAL) += serial_sti_asc.o
 obj-$(CONFIG_PIC32_SERIAL) += serial_pic32.o
-obj-$(CONFIG_STM32_SERIAL) += serial_stm32.o
 obj-$(CONFIG_BCM283X_MU_SERIAL) += serial_bcm283x_mu.o
 obj-$(CONFIG_BCM283X_PL011_SERIAL) += serial_bcm283x_pl011.o
 obj-$(CONFIG_MSM_SERIAL) += serial_msm.o