Fix .github

Removes outdated parts of our .github dir that were accidentally added back a few months ago. This removes:

- DCO notices
- issue templates
- PR templates

Author: yoshuawuyts

.github/CONTRIBUTING.md

@@ -54,10 +54,3 @@ contributors the benefit of the doubt and having a sincere willingness to admit
 that you *might* be wrong is critical for any successful open collaboration.
 
 Don't be a bad actor.
-
-## Developer Certificate of Origin
-All contributors must read and agree to the [Developer Certificate of
-Origin (DCO)](../CERTIFICATE).
-
-The DCO allows us to accept contributions from people to the project, similarly
-to how a license allows us to distribute our code.