Remove rtable dumping code from bootp.

This debugging code printing routing table data was introduced in rS25723,
22+ years ago. The last functional commit to this code was rS67534, 19 years ago.
The code has been turned off by default all this time.
Lastly, this code directly iterates radix tree and rtentries, which is not
not a proper interaction with routing system.

Differential Revision: https://reviews.freebsd.org/D24554