Remove "manual steps" from release guide.

release-guide.md contains most of the same instructions as build_release_candidate.sh ("(Alternative) Run all steps manually"). This is not ideal:

• Mirroring the instructions in release-guide.md doesn't add any value.
• Every single change to the process requires two identical changes to each file, and this makes it unnecessarily difficult to keep the two in sync.
• All the extra instructions make release-guide.md harder to read, obscuring information that the release manager actually does need to know.