Unset AB_OTA_UPDATER from BoardConfigMainlineCommon.

The previous need to build mainline devices as A/B no longer applies
[1], as we have unbundled the install of update-payload-key.pub.pem from
AB_OTA_UPDATER. Unsetting AB_OTA_UPDATER from BoardConfigMainlineCommon
defers the decision of using A/B OTA to be board-specific.

mainline_arm64 is considered as a specific board to build generic
system-only OTAs in A/B format. So it defines the two variables there.

[1] commit 571fab0610

Bug: 130516531
Test: TreeHugger
Test: Compare the built images for GSI and mainline targets. They don't
      have different artifacts due to the change.
Change-Id: I33cc9b6623dca7cc7c2a7e2c65cdbccced5e816e

target/board/BoardConfigMainlineCommon.mk

@@ -36,9 +36,6 @@ BOARD_AVB_ROLLBACK_INDEX := $(PLATFORM_SECURITY_PATCH_TIMESTAMP)
 
 BOARD_CHARGER_ENABLE_SUSPEND := true
 
-# Enable A/B update
-AB_OTA_UPDATER := true
-
 # Enable system property split for Treble
 BOARD_PROPERTY_OVERRIDES_SPLIT_ENABLED := true
 

target/board/mainline_arm64/BoardConfig.mk

@@ -28,6 +28,8 @@ include build/make/target/board/BoardConfigMainlineCommon.mk
 
 TARGET_NO_KERNEL := true
 
+# Build generic A/B format system-only OTA.
+AB_OTA_UPDATER := true
 AB_OTA_PARTITIONS := system
 
 BOARD_VENDORIMAGE_FILE_SYSTEM_TYPE := ext4