CONTRIBUTING: remove Pull Request bit

Also remove the "Also, please do not post patches on the issue
tracker" part, since that didn't seem to reduce the number of patches
inlined into bug reports. And now that we accept PRs, people will
probably try that first. We'll see.

Fixes #23779
Updates #18517

Change-Id: I449e0afd7292718e57d9d428494799c78296a0d2
Reviewed-on: https://go-review.googlesource.com/93335


Reviewed-by: Andrew Bonventre <andybons@golang.org>