drop real/legal name requirement from DCO (#4172)

Co-authored-by: Josh Simmons <git@josh.tel>

CONTRIBUTING.md

@@ -69,23 +69,8 @@ include the line in your commit or pull request comment:
 
     Signed-off-by: Your Name <your@email.example.org>
 
-We accept contributions under a legally identifiable name, such as
-your name on government documentation or common-law names (names
-claimed by legitimate usage or repute). Unfortunately, we cannot
-accept anonymous contributions at this time.
-
 Git allows you to add this signoff automatically when using the `-s`
 flag to `git commit`, which uses the name and email set in your
 `user.name` and `user.email` git configs.
 
-### Private sign off
-
-If you would like to provide your legal name privately to the Matrix.org
-Foundation (instead of in a public commit or comment), you can do so by emailing
-your legal name and a link to the pull request to dco@matrix.org. It helps to
-include "sign off" or similar in the subject line. You will then be instructed
-further.
-
-Once private sign off is complete, doing so for future contributions will not be required.
-
 [1]: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/creating-a-pull-request