Remove default --model-id argument

[alvarobartt]

What does this PR do?

This PR removes the default value for the --model-id in the clap CLI i.e., the entrypoint for Text Embeddings Inference (TEI), to prevent cases where the model is not captured correctly from e.g. an environment variable, and the BAAI/bge-large-en-v1.5 model is served instead without specific definition from the user, which can be misleading / confusing in some scenarios.

Note

Since this PR represents somehow a "breaking change", the PR should be released within a minor version i.e., 1.8.0 if within the next release.

What's missing?

• Once the PR is approved, update the --help outputs over the documentation and the README.md so that those are consistent with the latest CLI.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline, Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@Narsil