Remove 'systName' for grouping systematic variations

Closes https://gitlab.cern.ch/cp3-cms/bamboo/-/issues/92

As discussed there, the original reason (limiting number of lookups of nodes that change) to have this is not there anymore, due to other optimisations in bamboo.treeoperations.collectSystVars (a single traversal of the expression tree to find nodes with systematic variations, with the results cached for cuts and weights in Selections, and done once for plot variables and weights), and it's much less confusing which variations are correlated like this.

The tests (unit and regression) look fine, but more testing and feedback is welcome while I update the documentation (hence WIP).