refactor: miniooni should be outside of the engine (#206)

* refactor: miniooni should be outside of the engine

This is part of https://github.com/ooni/probe/issues/1335. We also need
to think whether we wanna keep libminiooni and miniooni separated.

The previous use case for having a top-level libminiooni was that of
enabling others to integrate miniooni into other binaries.

This was usegul when studying internet censorship in Spain in May 2020.

I am wondering whether we should be keeping this complexity. I am not
sure about this and probably we should be killing it.

(In any case, reducing complexity is not the objective of this diff,
since I would like instead to move things around with minimal changes
and make sure we have a ~good repository organization here.)

* fix: import in libminiooni

internal/engine/cmd/miniooni/README.md

@@ -1,8 +0,0 @@
-# miniooni
-
-This directory contains the source code of a simple CLI client that we
-use for research as well as for running QA scripts. We designed this tool
-to have a CLI similar to MK and OONI Probe v2.x to ease running Jafar
-scripts that check whether these tools behave similarly.
-
-See also libminiooni.