Create new endpoint `/api/v1/environments/:key/delete-segment-override/`

[matthewelwell]

Similar to #6233 , we need to create a new endpoint at /api/v1/environments/:key/delete-segment-override/ which will allow us to have a single endpoint to remove a segment override, regardless of whether the environment has v2 feature versioning enabled or not.

The payload should look like the following:

{
  "segment": {
    "id": 1
  }
}