handbook: Drop the git mailing lists list from Mirrors

[trasz]

Summary

Just reference the right section; no need to duplicate all this
information here.

[debdrup]

I don't think we should be removing references to these mailing lists as I know for a fact that I'm not the only one that uses them or might need them in the future.

[trasz]

You're definitely not the only one, however, is the Mirrors chapter really the best place to put them? Given that they are already described in the chapter dedicated to mailing lists?

[debdrup]

That's fair - perhaps link to it instead?

[trasz]

But there's already a link to it...?

[pauamma_gundo.com]

LGTM.