um: Remove obsolete pcap driver

Remove the pcap driver in UML. It is obsolete. It does not build on
recent systems due to changes in libpcap and its dependencies.

The vector driver's raw transport in UML provides identical
functionality.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Link: https://patch.msgid.link/20240328132424.376456-1-anton.ivanov@cambridgegreys.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>