Remove Caine's Requirements from CONTRIBUTING.md

[targos]

I read some discussions (#117, #26) and it is not quite clear to me if Caine or another bot will ever come back to help managing the issues.

Anyway, that last section of CONTRIBUTING.md currently doesn't make sense and it can be confusing for the new contributors.

Shouldn't it be removed for now ? If so, I can make the PR.

[bnoordhuis]

A PR would be great.

[bnoordhuis]

Fixed by e80f803.