Remove unused leftovers
The following discussion from !3527 (merged) should be addressed:
Reviewing this MR revealed a few interesting findings:
there are some files which appear to be unused leftovers, e.g.
bin/rndc/include/rndc/os.h- they are not included in source tarballs produced by
make distand yet these compile just fine,
some files tracked by Git are of questionable use, e.g.