replmd: Small refactor to replmd_check_singleval_la_conflict()

replmd: Small refactor to replmd_check_singleval_la_conflict()

Now that the code is all in one place we can refactor it to make it
slightly more readable.

- added more code comments
- tweaked the 'no conflict' return logic to try to make what it's checking
  for more obvious
- removed conflict_pdn (we can just use active_pdn instead)
- added a placeholder variable and tweaked a parameter name

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13055

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>