remove the markdown related switches

[thewilsonator]

This has been enabled by default for a couple of years now.

Might also need a spec PR, e: there doesn't seen to be any need for this. The spec only mentions the features.

https://forum.dlang.org/post/bcwupfeuaghvjzyunbng@forum.dlang.org

[dlang-bot]

Thanks for your pull request and interest in making D better, @thewilsonator! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#14050"

[ibuclaw]

Should reverts be deprecated first?

[thewilsonator]

Dunno. Part of this PR was to figure out what, if anything, breaks. Buildkite is green so probably not much.
If this were a switch that affected codegen I'd be much more cautious, but this affects documentation only, and it's been enabled by default for several years now.

[ibuclaw]

Should transitions be deprecated first?

[thewilsonator]

ditto

[thewilsonator]

Hmm, I wonder if this comment is still accurate?

[ibuclaw]

Hmm, I wonder if this comment is still accurate?

I assume the order would have been:

1. Undocument the preview in dmd
2. Remove the preview from the phobos makefiles
3. Deprecate the preview.

1 and 2 are interchangeable (and in-fact if 2 is merged first, then 1+3 can be done together).

[thewilsonator]

Remove the preview from the phobos makefiles

It appears to already have been removed.

[thewilsonator]

Added changelog entry.

@WalterBright seems to be in favour of this: https://forum.dlang.org/post/t4mhhc$1blq$1@digitalmars.com

[RazvanN7]

I think this is fine. I find it hard to imagine that anyone goes through the pain of using -revert=markdown