fdo#34170 Disable deleting in the native Open Dialog

Change-Id: I0ccd4b06b2db399711004c3e6505a28353604aeb
Reviewed-on: https://gerrit.libreoffice.org/6892
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>