Revert "machined: don't force terminate registered machines"

This reverts commit 206e7a5f7b55ac61188efd895e65ab26e478cbb2.

We actually want to allow shutting down containers that use
RegisterMachine() rather than CreateMachine() to register their own
unit. It should be safe to do so, since the primary usecase for
RegisterMachine() are container managers that run only a single
container within their own unit, such as systemd-nspawn.