Removed method of effective diffusivity in Fickian diffusion algorithm.
Removed method of effective diffusivity in Fickian diffusion algorithm.

It was an old code and we need a general method for both diffusivity and
permeability.


Former-commit-id: a24f5249b6b375626d62448671a345974888e8ef
Former-commit-id: 4b7f4296feae8274ed4809a64d9b0903456b5152
amdouglas/OpenPNM,amdouglas/OpenPNM,TomTranter/OpenPNM,PMEAL/OpenPNM,stadelmanma/OpenPNM