ldb-samba: remove unused ldif_comparision_objectSid_isString()

ldb-samba: remove unused ldif_comparision_objectSid_isString()

This is unused because it does things badly, by just guessing and
not allowing valid sids that start with "s-". All the places that used
to use it were calling ldif_read_objectSid() or similar which correctly
check for string SIDs by actually trying to parse them. That begins
with looking for the "S-"/"s-", so this shortcut is not saving any real
work.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=10763

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