endpointmanager: remove lookup by container-name

Remove the deprecated container-name: endpoint ID lookup path now that
Docker plugin support is gone (cf. commit f461a50 ("chore: Remove
docker-plugin") and nothing populates the field anymore.

Also remove some already unused legacy test helpers referencing the
container-name field.

Signed-off-by: Tobias Klauser <tobias@cilium.io>

Author: tklauser