wlandebug.8: remove Xref for missing manpages

Remove references for: athdebug(8), athstats(8) and wlanstats(8)

Those are tools in the tools/ directory that are not built as part of the base
system. According to the toolds/README file:

"...these tools are not meant to be built as part of the standard system..."

Even more, the tools/tools/README is not udpated and wlanstats does not even
built on current:

error: cast from 'struct sockaddr *' to 'const struct sockaddr_dl *' increases
required alignment from 1 to 2 [-Werror,-Wcast-align]

PR: 227174
Reported by: freebsd.org@alexandrews.me.uk
Reviewed by: gbe@ adrian@
Approved by: gbe@ (mentor) adrian@
Differential Revision: https://reviews.freebsd.org/D29033