MFC r363141:

Don't print VNET pointer when initializing dummynet

When dummynet initializes it prints a debug message with the current VNET
pointer unnecessarily revealing kernel memory layout. This appears to be left
over from when the first pieces of vimage support were added.

PR: 238658
Submitted by: huangfq.daxian@gmail.com
Reviewed by: markj, bz, gnn, kp, melifaro
Approved by: jtl (co-mentor), bz (co-mentor)
Event: July 2020 Bugathon
Differential Revision: https://reviews.freebsd.org/D25619