vm no longer measures bogoclock or times, relies on os for that

vm no longer measures bogoclock or times, relies on os for that

* libguile/vm.h (struct scm_vm): Remove "time" and "clock" members. The
  time was bogusly measured, and the "clock" measured instructions
  retired, which is not a very useful measurement, and it was causing
  lots of memory accesses. Not that I have done a proper profile,
  though...
  (scm_vm_stats): Remove this procedure, which provided access to "time"
  and "clock".

* libguile/vm.c:
* libguile/vm-engine.h:
* libguile/vm-engine.c:
* libguile/vm-i-system.c: Adapt to scm_vm changes and scm_vm_stats
  removal.

* module/system/repl/command.scm:
* module/system/vm/vm.scm: Adapt to vm-stats removal by removing
  vm-stats from <repl>.