Revert correlated shift functionality

Created by: Zaharid

This was broken on some code paths, seemed to incorrectly duplicate the computation on others, and it is affected by very significative conceptual limitations (see e.g. https://github.com/NNPDF/nnpdf/pull/825#issuecomment-655331325) while it is perhaps not so obvious what problem did it solve in the first place (see e.g. https://github.com/NNPDF/nnpdf/issues/506#issuecomment-509213007).

It was not used in the 4.0 paper, and as a matter of empirical fact, that did not cause the galaxy to be sucked into a black hole. The coding related aspects could be improved upon, but that could benefit from a fresh start. The algorithmic code lives in history and it is easy enough to recover if needed.

Closes #1171 (closed).