Catch mkheaders.c up to the removal of counted device support in 2005.

mkheaders.c hasn't made headers in ~15 years. Belatedly update the comments to
reflect that all it does these days is warn about 'device foo' lines in the
config where we don't know what a 'foo' is.

Remove extra includes too. These also haven't been needed for 15 years and
weren't removed at the time the comment wasn't updated...