Revert "Make ab_ota_partitions read only"

This reverts commit 929f1ad1d63c9fdd286bde9a4de26b583412498e.

Reason for revert: Caused a build break
Bug: 175633875

Change-Id: I6cb518a574c865a2e2c4ecdc4203dfa03bbf05d9
diff --git a/core/board_config.mk b/core/board_config.mk
index 36dd7d8..1a80348 100644
--- a/core/board_config.mk
+++ b/core/board_config.mk
@@ -53,7 +53,6 @@
   TARGET_NO_RADIOIMAGE \
   TARGET_HARDWARE_3D \
   WITH_DEXPREOPT \
-  AB_OTA_PARTITIONS \
 
 # File system variables
 _board_strip_readonly_list += \