KCC: simplify repsfrom pruning in KCC.translate_ntdsconn()

KCC: simplify repsfrom pruning in KCC.translate_ntdsconn()

This section is actually quite simple -- we don't want a repsFrom if
there is no DRS connection -- but the existing comments and code had
managed to cause us hours of confusion and much gnashing teeth, the
evidence of which will hopefully have been squashed out of the git
history by the time you read this.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>