fdo#75757: remove inheritance to std::vector

from LayeredDevice::LayerContainer.

Change-Id: Icc4b16db92daadbd742e83e90c95477c0c465f5b
Reviewed-on: https://gerrit.libreoffice.org/11397
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>