Remove Multi-threaded image conversion for fast image processors
Author: yonigozlan
Pull request: yes

# What does this PR do?

Looks like this was introducing a lot of overhead, and slowing processing especially when processing a single image.
I'm removing this for now and I'll try to find a better way to parallelize this in the future.
This should hopefully solve the recent flakiness of `test_fast_is_faster_than_slow`
@ArthurZucker @ydshieh @Cyrilvallez 

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_36105). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

As @ydshieh there might be a tradeoff to find between number of input images and when to trigger thread pool !˜


Maybe change the test slightly for now would be the best decision @yonigozlan (maybe have two different tests for ingle and multiple items) ? It would be great to have this merged asap as the CI is flaky and I had to restart the tests multiple times to make it pass in another PR. 

Merging this for now as it seems to be a big issue for CI. I'll experiment with finding a tradeoff/adapting the test for multi-threading later on.

@yonigozlan we should keep the thread pool IMO and update the test otherwise we are breaking performances for people that relied on it!