bug: remove generation values, when deleting a site

[peterdudfield]

Current when deleteing a site, the generation values are not removed

https://github.com/openclimatefix/pv-site-datamodel/blob/main/pvsite_datamodel/write/user_and_site.py#L303

It would be good to remove this generation values too

[peterdudfield]

We are pausing this issue in this repo. We will be moving over to a new data platform. More information to come on this.