Code review comment for ~goneri/cloud-init:freebsd_mount_sync

mount_cb's rw flag is always False. This is actually what leads my to thing the `sync` flag is pointless. I'm convinced we can drop the two parameters.

In addition, the current code base relies on blkid which is Linux specific. To get cloud-init to work on FreeBSD, we need to mock blkid. In addition, there is no Linux 'distro' beside FreeBSD. So I doubt such change will impact anyone.

From my view it's a good opportunity to clean up some old irrelevant function parameters and simplify the whole code base.