Bluetooth: Mesh: Remove redundant 'provisioned' variable

This variable is both redundant as well as problematic when it comes
to adding persistent-storage-based provisioning information, which
will not come through main.c or the bt_mesh_provision() API. Just
remove it and use bt_mesh.valid which serves the same purpose in
practice.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>