dummynet: Remove unused code

We never set 'busy' and never dequeue from the pending mq. Remove this
code.

Reviewed by: ae
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30313

(cherry picked from commit 02c44f40f984951fe34a1c5a43f40ff8147c52ca)