[RF] Remove HistoToWorkspaceFactory (non-Fast)

The original `HistoToWorkspaceFactory` produced models that consisted of
a  Poisson term for each bin.  In this "number counting form" the
dataset has one row and the collumns corresponded to the number of
events for each bin. This led to severe performance problems in
statistical tools that generated pseudo-experiments and evaluated
likelihood ratio test statistics.

Nowadays, everyone uses the faster `HistoToWorkspaceFactoryFast`
implementation that  produces a model in the "standard form" where the
dataset has one row for each event, and the column corresponds to the
value of the observable in the histogram.

Therefore, the original `HistoToWorkspaceFactory` is now removed to
avoid  confusion and maintainance burden.