arm: stm32: Remove redundant thumb build selection

All armv7m arch builds are thumb & SYS_THUMB_BUILD is already selected by
CPU_ARMV7M.
Signed-off-by: NVikas Manocha <vikas.manocha@st.com>

arch/arm/Kconfig

@@ -1266,7 +1266,6 @@ config STM32
 	select CPU_V7M
 	select DM
 	select DM_SERIAL
-	select SYS_THUMB_BUILD
 	imply CMD_DM
 
 config ARCH_STI