fdo#75757: remove inheritance to std::map

from ItemFormatMap.

Change-Id: I956b5797e677d22eb71fe801b650db7c982d6d51
Reviewed-on: https://gerrit.libreoffice.org/11854
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>