PCIe HotPlug: remove tests for DL active link capability

As of r313097, the HotPlug code requires the link to support
reporting of the data-link status. Remove tests for this capability
from code that can now assume its presence.

Suggested by: jhb
Reviewed by: jhb
MFC after: 3 days
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D9431