arm64: STM32: drop an undefined Kconfig symbol

Drop ARM_SMC_MBOX since it is not defined or used anywhere else
in the kernel source tree.

Fixes: 9e4e244 ("arm64: introduce STM32 family on Armv8 architecture")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20251230181429.3429404-1-rdunlap@infradead.org
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

Author: rddunlap

arch/arm64/Kconfig.platforms

@@ -372,7 +372,6 @@ config ARCH_STM32
 	bool "STMicroelectronics STM32 SoC Family"
 	select GPIOLIB
 	select PINCTRL
-	select ARM_SMC_MBOX
 	select ARM_SCMI_PROTOCOL
 	select REGULATOR
 	select REGULATOR_ARM_SCMI