Removal of `-transition=vmarkdown` and `-revert=markdown`

[Nicholas Wilson]

Hi all

I plan on removing `-transition=vmarkdown` and 
`-revert=markdown`[1] and making markdown replacement in DDoc 
always on. Markdown replacement in DDoc has been the default 
since Sep 2020. All projects on buildkite compile with this 
change.

Are there any objections to this? This could go through a 
deprecation if necessary, but given this change only affects 
documentation generation, I don't think this is necessary. The 
small sample that is buildkite does not fail on any projects, 
suggesting this switches are very rarely deployed, if at all.

Nic

[1]: https://github.com/dlang/dmd/pull/14050