Clean up api.py file

[LexABzH]

Problem

This file includes unused stuff (such as the PreProcessor class) that we can probably remove.
We also can improve preprocess_pipeline. Maybe split by type of input data to make it clearer ?
Also, DEFAULT_PIPELINE should probably be changed to something simpler.

Solution

• Get rid of unused stuff.
• Make preprocess_pipeline clearer
• Change DEFAULT_PIPELINE to something simpler.

Additional context

Adapt tests !