🚨🚨🚨 [`Blip`] Refactor the Blip modeling file + test file 🚨🚨🚨
# What does this PR do?

Removes the file `test_modeling_blip_text` as its content is totally duplicated inside `test_modeling_blip`, so that we avoid running these tests twice

This PR also refactors the modeling file of `blip`, to have a single file for the whole architecture. I also realized that there is no necessary to have a `BlipTextPretrainedModel`, so I decided to remove that class for a cleaner implementation. Hence, this PR might introduce breaking changes for blip.

cc @ydshieh 
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_22648). All of your documentation changes will be reflected on that endpoint.<|||||>Let's wait our great @sgugger to express his opinion on if we are allowed to change this. 
I think it's fine however.<|||||>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.