Revise tech talks

[vmarkovtsev]

https://github.com/src-d/guide/blob/master/developer-community/tech-talks.md describes how we did tech talks in 2017. We no longer do them, so how about changing present to past tense everywhere.
cc @campoy

[dpordomingo]

Or maybe archive that guide.

[campoy]

Yeah, I agree this should be simply removed. I'll send a PR.

[campoy]

Actually, I think it'd be better to take some time and review the content of the whole folder.

I'll work on this later on, as it's not really urgent.

[dpordomingo]

Who should be in charge of reviewing the whole section now that @campoy is no longer with us? :(
@vcoisne do you have any idea?

[campoy]

You made it sound like I'm dead! 💀🤣

[vcoisne]

@dpordomingo we should review the content of the whole folder. I'll add to my to-do list, not a priority though.

[smola]

For the purpose of this issue, we may start adding a bold header to the tech talks guide, making it clear that it's inactive / historical.
cc @dmitryvinn