Remove cloud-deploy deprecated image

This MR removes:

• the section in the README about the deprecated cloud-deploy image.
• the build process for this deprecated image.

Closes gitlab#213386 (closed)

Manual testing

Image is still in registry:

> docker pull registry.gitlab.com/gitlab-org/cloud-deploy:latest
latest: Pulling from gitlab-org/cloud-deploy
d51af753c3d3: Pull complete 
fc878cd0a91c: Pull complete 
6154df8ff988: Pull complete 
fee5db0ff82f: Pull complete 
3072691d12c7: Pull complete 
4fe55dbbee84: Pull complete 
ace1bf8071bb: Pull complete 
30fa375b5481: Pull complete 
4b845ee8acb8: Pull complete 
fb1d4c8b73b9: Pull complete 
Digest: sha256:dd35094ca795fa6b27be22dadd08388229a28eea8922622930e142da6e52cfd4
Status: Downloaded newer image for registry.gitlab.com/gitlab-org/cloud-deploy:latest
registry.gitlab.com/gitlab-org/cloud-deploy:latest