rc6 web list of documents includes broken links to network guide

Summary

https://ftp.rtems.org/pub/rtems/releases/6/rc/6.1-rc6/ includes this section which references non-existent documents. Remove these. We do not package legacy networking or this manual anymore.

RTEMS Networking User Manual
docs/html/networking
docs/rtems-6.1-rc6-networking.pdf
docs/rtems-6.1-rc6-networking-html.tar.xz

Steps to reproduce

Visit the above URL

Pre-set options