remove bind10 leftovers

@godfryd came up with a clever script that lists files that are in repo, but not in tarball. Here are files to be removed:

• doc/guide/STYLEGUIDE (last edited by Jeremy Reed, 9 YEARS AGO) (already removed)

• doc/design/datasrc/* (bind10 data source, talks about shared memory) (already removed)

• src/bin/sockcreator (there's capabilities mechanism in Linux now)

• src/lib/asiolink/doc/* (2 jpegs, explaining auth/recursive DNS operations)

• socket session doc: https://jenkins.isc.org/job/Kea_doc/doxygen/dc/d78/SocketSessionUtility.html