Remove hook_fundraiser_get_credit_encryption_path

When fundraiser_sustainers is looking for the sustainer key file, it asks other modules that implement this hook to return a path to the directory where the file lives. The only module to implement this hook is fundraiser_commerce, which is not an explicit dependency.

So during simpletests, fundraiser_sustainers can be enabled without any module implementing this hook and we get an undefined offset and no sustainer key path. As a result the sustainers tests have to enable both modules and we get the extra overhead of fundraiser_commerce.

Since it looks like we didn't make additional use of this hook, my proposed solution is to remove it and let fundraiser_sustainers find its own sustainer key path. It's just doing a variable_get() from the encrypt key path anyway.