Remove BOARD_USES_RECOVERY_AS_BOOT from aosp_arm64_ab

The target does not have a kernel so this flag does
not make sense.

Bug: 37443196
Test: flash aosp_arm64_ab system.img on sailfish
Change-Id: I1f22866dd5ecdd16615c02fdc8aa898b1ec2ddbd
diff --git a/target/board/generic_arm64_ab/BoardConfig.mk b/target/board/generic_arm64_ab/BoardConfig.mk
index 2c3821a..cf79019 100644
--- a/target/board/generic_arm64_ab/BoardConfig.mk
+++ b/target/board/generic_arm64_ab/BoardConfig.mk
@@ -64,7 +64,6 @@
 
 # Enable A/B update
 TARGET_NO_RECOVERY := true
-BOARD_USES_RECOVERY_AS_BOOT := true
 BOARD_BUILD_SYSTEM_ROOT_IMAGE := true
 
 # TODO(jiyong) These might be SoC specific.