Use potential-shift as default modifier

Use potential-shift as default modifier

The potential-shift-verlet modifier is no longer useful, as
potential-shift was the default with the only cut-off scheme
remaining. That is now explicit. The former modifier is now
named to reflect that it doesn't do anything.

The documentation no longer mentions that modifier, but if a user
supplies an .mdp file using it, grompp silently adopts the behaviour
of the potential-shift modifier. No .tpr files contain that value, as
grompp always converted it to something else, but in any case this
patch doesn't change the situation if somehow such a .tpr does exist.

Change-Id: Ie4f848a298750b9749ebb083812bad06b2faa45c