Revert "convert diagrams mentions to diagrams/Core team"

It appears this does not actually work; see
https://github.com/fpco/stackage/pull/244#issuecomment-48469406 .

This reverts commit bf61e01909d788146909602b07cf8e30e32cf69e.
Merge pull request #255 from byorgey/master

Revert "convert diagrams mentions to diagrams/Core team"