[scd/isa] cleanup ISA repo implementation(s), notably IsaRepoV3

[Shastick]

The isaRepoV3 struct has the following comment:

// The purpose od isaRepoV3 is solely to support backwards compatibility
// It will be deleted from the codebase when all existing production deployments have been upgraded to 3.1.0+.

This means that we might be able to clean up some code. (Ie, identification_service_area_v3.go and possibly subcriptions_v3.go)

This implementation of the repo is used if the database schema has a version that is smaller than 4.0.0

@BenjaminPelletier the comment above was added approximately 4 years ago: how likely is it that all real-world deployments that still pull in the latest updates of the DSS would still run on an old schema?

Knowing whether or not to get rid of this older implementation is relevant for some work happening for #814