fdo#75757: remove inheritance to std::vector

from DdeItemImp.

Change-Id: Idb26f982f96ddf1641fd8030d87c88b0cbc14f16
Reviewed-on: https://gerrit.libreoffice.org/11482
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>