fdo#75757: remove inheritance to std::map

from oox::core::Relations.

Change-Id: If2e0109a2ad6598436177b7638cb6d568fb2d3d6
Reviewed-on: https://gerrit.libreoffice.org/11899
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>