Delete actors.md

[Khalian]

Based on follow up conversations in #2542

Specifically this comment

Thanks!

Moreover, is there a reason that we are keeping the main Actors page?

https://docs.scala-lang.org/overviews/core/actors.html

I think there is a value of removing stuff that could potentially confuse new users. There are possibly more pages like this one that shall be removed.

[julienrf]

Thank you @Khalian!