Remove the sysctl kern.allowdt code from kernel if dt(4) is not

Remove the sysctl kern.allowdt code from kernel if dt(4) is not
configured.  This will result in a "value is not available" error
from sysctl when trying to enable dt on a kernel without support.
The variable allowdt should be in the device, not in sysctl source.
We don't need #ifdef for extern and prototypes.
OK mpi@