images: Drop explicit libssh

This has been unnecessary since commit b06bd37, as installing
cockpit-bridge brings this in as a dependency. It will actually drop
from the image after the next Cockpit release, as it was dropped in
cockpit-project/cockpit#19441

images/scripts/debian.setup

@@ -21,7 +21,6 @@ libpcp3 \
 libpolkit-agent-1-0 \
 libpolkit-gobject-1-0 \
 libpwquality-tools \
-libssh-4 \
 libteam-utils \
 libvirt-daemon-system \
 libvirt-dbus \