ci: Drop the release workflows before merging into main

`cran-submission`, `win-builder` and `release-gate` were a trial on this branch,
and their adoption is still open in cynkra/cynkratemplate#147.
`main` keeps the `.github/` it gets from the template until then.
The files remain at the `v2.3.4` tag for the next `cran-*` branch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019iwyUfQ6pFAhL3jUUocHFm

Author: claude