Simplify selection of indices in FlaxCLIPTextTransformer to support JAX model conversion tooling
# What does this PR do?
It makes a simple change so that `FlaxCLIPModel` compatible with the `jax2tf` conversion tool, which currently doesn't support certain types of gather operations. It's explained by a JAX engineer here: https://github.com/google/jax/issues/9572#issuecomment-1047162054

## Who can review?
@patil-suraj 

Note that I haven't ran any tests. Please feel free to close this if testing is required on my part, or if there simply isn't interest in making this change at this point (`jax2tf` will eventually support a larger subset of gather operations, so it's not super high-priority in that sense).

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15762). All of your documentation changes will be reflected on that endpoint.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.