Remove unused leftovers

The following discussion from !3527 (merged) should be addressed:

• @michal started a discussion: (+2 comments)

  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 dist and yet these compile just fine,

  • some files tracked by Git are of questionable use, e.g. bin/rndc/rndc.conf.