Remove unused GetFilesetID method.
Remove unused GetFilesetID method.

The last reference was removed in revision cbc18970d5 ("Re-applied the
path-storage fix as the merge failed somehow").

duplicati/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati