fs_mgr: deprecate check for ro.build.system_root_image

If there is a "/" in the fstab, can only be there on system as root.

This drops the troubling and annoying log message:

    [libfs_mgr]Could not find verity device for mount point: /: No such device or address

Test: manual
Bug: 109821005
Change-Id: I6bbfc0af63e1526e52a1c6365241dbc1d5ee7c6a