Remove references to BOARD_BUILD_SYSTEM_ROOT_IMAGE
This variable is always false or empty now.
Remove it so we can mark it as deprecated later.
Bug: 241346584
Test: Presubmit
Change-Id: I895634d4a20c7bd1cd2b3712917e7c1aae542b5d
The code in this directory is experimental. Bazel support for Android Platform is undergoing active development and workflow stability is currently not guaranteed.