Remove experiment hash

What does this MR do?

Get rid of EXPERIMENTS hash

• This removes the experiment hash, which gave us a lot of merge conflicts recently and puts us into a better position for more experiments
• It aligns the idea of experiments more like with gitlab-experiment where each experiment is its own class
• The idea of the alignment is that at some point we can move easily to gitlab-experiment gem under the hood
• Rename Experiment to LegacyExperiment to make it clearer in which direction we're heading to

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team