WIP: Revert "Adds experimental blocks flag for blocks that may not be ready for general use (#64121)"

[t-hamano]

What?

This PR reverts the experimental flag for blocks added in #64121.

Note

Note: This PR is not guaranteed to be merged, but was prepared in advance to prevent #64121 from shipping in Gutenberg version 19.0.

Why?

#64121 was added for a reason:

Blocks in development may not be ready for general use. By having users opt in to an experiment to test these blocks, we can iterate without worrying about writing multiple block deprecations.
Once a block is stable, it can be promoted from the experiment to being included by default in block-library.

However, the following opinion exists:

A single block is usually well scoped and when it's quite close to land, but we're not sure if it's going to be stable for a WP release, we have the "__experimental" flag in block's metadata (block.json). In all other cases, the block should be tested in the respective PR and not add extra unfinished code in the repo.

We probably shouldn't add such an option until we have a clear rule of ​​how to handle "experimental" blocks in the Gutenberg plugin.

[t-hamano]

Based on the discussion in 64121, I'm closing this PR.