min_site_separation_ is unused outside of ChooseInitialSites

[jnyrup]

I noticed that since c84fc2a s2builder.cc stopped using min_site_separation_ outside ChooseInitialSites.
If that's on purpose then min_site_separation_ could be a local variable inside inside ChooseInitialSites instead of class scoped.

[jmr]

Thanks. I've fixed it internally.