The functionality for block device DAX was already removed with this
commit acc93d30d7d4 ("Revert "block: enable dax for raw block devices"")
However, we still had a config option hanging around that was always
disabled because it depended on CONFIG_BROKEN. This config option was
introduced in this commit:
commit 03cdadb04077 ("block: disable block device DAX by default")
This change reverts that commit, removing the dead config option.