Remove unused `add_ephemeral_model_prefix` method

### Housekeeping

- [X] I am a maintainer of dbt-core

### Short description

This method appears to be **unused** across dbt-core, dbt-common, and dbt-adapters:

https://github.com/dbt-labs/dbt-core/blob/c668846404c78ea7317dc0523b49703c61c9885f/core/dbt/utils.py#L140-L141

Instead, [this](https://github.com/dbt-labs/dbt-adapters/blob/e3465c2a7baaea66f71b1da94647a193c281a992/dbt/adapters/base/relation.py#L235-L236) is the method that is in-use:

```python
def add_ephemeral_prefix(name: str):
return f"__dbt__cte__{name}"
```

### Acceptance criteria

Remove this method.

### Suggested Tests

1. Confirm that existing CI passes
1. Confirm that is is not used within each relevant repo with a code search like `git grep "add_ephemeral_model_prefix" .`

### Impact to Other Teams

Should have zero impact to other teams.

### Will backports be required?

No backports required.

### Context

_No response_