Remove legacy elasticsearch images

[jmbowman]

A/C

• Devstack no longer references elasticsearch or elasticsearch7 images
• If there is a reason we keep them around, remove the dependency

docker-compose.yml still includes elasticsearch and elasticsearch7 images even though all the services have already switched to using the elasticsearch710 image. Remove these older images from the file unless you find some obscure reason we need to keep them around.

[rgraber]

#1163