[`Blip`] Remove redundant shift right
# What does this PR do?

Fixes https://github.com/huggingface/transformers/issues/23000

In fact `_shift_right` does not need to be called inside `BlipForQuestionAnswering` as the right shifting of the tokens is done directly on the text decoder as mentioned by the user. Therefore, that class will be trained to perform next-next token prediction instead of next-token prediction.

The fix is to simply remove that shift method and the call to it.

cc @sgugger 
_The documentation is not available anymore as the PR was closed or merged._<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23153). All of your documentation changes will be reflected on that endpoint.