Remove broken and undocumented "demo mode" feature

[ryanahamilton]

After fixing and refactoring the "demo mode" feature in #14595, I feel that it would be more productive to replace that PR with this one that removes the feature instead.

• It has very little documentation
• It has been largely broken for an unknown period of time without any issue reports. I only encountered it because I wanted to refactor its implementation.
• As illustrated by @ashb it doesn't do a really great job if someone were to capture a screenshot from remote demonstration.
• As pointed out by @HaloKo4, only being able to activate it from the config doesn't make it a very convenient feature that can easily be enabled/disabled.

If this is a feature people would like to see, I think it would be better to see a new implementation proposed that addresses these shortcomings.

[ashb]

I think given its been broken for a while without reports, and the ability to unblur via GANs we should likely remove i let's give people a bit of a heads up on the dev mailing list end find them say, 3 days to object/propose something else.

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest master at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[bbovenzi]

If it's not all that useful, then I'm in favor of cleaning up at least a bit of the FE code.

[xinbinhuang]

I suggest that we remove it. I only came across this when I was digging around the codebase, and have never heard of people using or talking about it previously. Mostly, I have seen people just creating some dummy dags for illustration and presentation purpose..

[ryanahamilton]

I put a call out to the Dev list as a gut check on this and did not receive any dissenting feedback. Going ahead and merging this PR (removing the "demo mode" feature).

Failed checks are unrelated.