chore: remove ietf/group/colors.py, which is unused

[larseggert]

No description provided.

[rjsparks]

This feels like the last parts of a notable step backwards, even if we aren't going to use these particular colors.
We had been on our way to normalizing the color choice across views, particularly between the agenda and the agenda editor. Has this normalization happened a different way, or are we back to different choices being made in different parts of the code?

[larseggert]

Colors should be managed as part of the CSS, rather than via the Python code?

[rjsparks]

that's reasonable - but the problem I'm pointing to (which isn't really the responsibility of this PR, but of changes leading up to it) then exists with how views use the css. I think we're diverging (Nick's agenda pages use the base colors, but darken them in a wash for instance). Have we reconciled group colors between the agenda and agenda editor yet?