Remove `demo` capacity from Playbook

[rosiel]

Two issues:

• the branch of islandora-sandbox being pulled is install-profile, which includes components which only work with php 7.4. This will be solved by Use "main" branch of islandora-sandbox for Install Profile ("demo"). #255
• We stopped installing bartik in Don't enable bartik. #259 which is required by some config in islandora_defaults.

We could fix this, or deprecate "install a demo" in favour of the Docker/portainer-based demo. I'm not sure that anyone is recommended to use the install profile at this point if they're starting a site.

[rosiel]

I'm going to vote that we remove Demo from playbook.

[rosiel]

WIP: https://github.com/Islandora-Devops/isle-dc/compare/development...rosiel:isle-dc:remove-demo?expand=1

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