fdo#75757: remove inheritance to std::map

from UniqueIndexImpl.

Change-Id: Iaa9040dff117ed5b05955c9f6eef31878dccf3b0
Reviewed-on: https://gerrit.libreoffice.org/11951
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>