zenfone9: 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
diff --git a/device.mk b/device.mk
index 6a39479..c0df1c3 100755
--- a/device.mk
+++ b/device.mk
@@ -84,9 +84,8 @@
     AntHalService
 
 # Api
-BOARD_API_LEVEL := 31
-BOARD_SHIPPING_API_LEVEL := $(BOARD_API_LEVEL)
-PRODUCT_SHIPPING_API_LEVEL := $(BOARD_API_LEVEL)
+BOARD_SHIPPING_API_LEVEL := 31
+PRODUCT_SHIPPING_API_LEVEL := $(BOARD_SHIPPING_API_LEVEL)
 
 # Atrace
 PRODUCT_PACKAGES += \