Phase out "enableFitGradients no" from the configuration file

[HanatoK]

As discussed in the Google Chat group, the users should not disable the calculation of fit gradients. However, some reference data are generated with "enableFitGradients no" when the fit gradients of the vector components are not available, so disabling "enableFitGradients no" woulld require to re-generate the reference files for GROMACS, LAMMPS and NAMD.

[giacomofiorin]

I think it makes sense to regenerate, i.e. effectively remove the reference data produced without fit gradients.

The current version of the repository should not have to track features that are no longer supported and, more importantly, not useful in production.