sm8450-common: Don't set BOARD_API_LEVEL

build/make/core/board_config.mk:1001: error: BOARD_API_LEVEL must not
set manully. The build system automatically sets this value..

Change-Id: If59ae9dea6cb13701d7217a1c0a355d4e07113eb

common.mk

@@ -46,7 +46,6 @@ PRODUCT_PACKAGES += \
 
 # API
 PRODUCT_SHIPPING_API_LEVEL := 31
-BOARD_API_LEVEL := 31
 BOARD_SHIPPING_API_LEVEL := 31
 
 # Atrace