[dmctl] Remove virtcon channel from devcoordinator.

We have already switched to using a more direct path to create virtcons,
this removes the old path through /dev/misc/dmctl to the devcoordinator.

Issue: ZX-3403
Test: Booted frank, virtcon still works, this change is in essence a
no-op as nothing is using this code.

Change-Id: Icb0b77984a818a56f98d5099d111878e9f88ea57