* live-build/ubuntu-cpc/functions: Add a function, teardown_mountpoint,
to reverse the work done in setup_mountpoint. Lack of this function
has forced users of setup_mountpoint to implement this separately
and the implementations have diverged. (LP: #1716992)
* live-build/ubuntu-cpc/functions: Remove umount_settle function.
The was only used where teardown_mountpoint was lacking.