Remove "Reviewers:" for spr land

[MaskRay]

My typical workflow: compose a patch, run spr diff to create a PT, add reviewers on GitHub, get approved.
When I run spr land, the landed commit will include both Reviewers: and Reviewed By:. Reviewed By: is useful, but I want to hide Reviewers:.

When we were using Phabricator for llvm-project code review, the community consensus was that these tags array("Subscribers", "Reviewers", "Tags") are not useful. So there was a local patch to filter out them MaskRay/phabricator@5d531bc

[arichardson]

I just started using spr for LLVM development and also noticed the extra Reviewers: field. I agree that it's not useful and would like to see an option to omit it.

[arichardson]

I just had a look at the forks of this repository and it seems like master...bogner:spr:succint-messages does almost what I'd like to see (but I'd actually like to keep Reviewed by).