Remove Finder

[tholor]

Proposed changes:
We deprecated Finder a while ago and replaced it with the more flexible Pipeline. Let's now get finally rid of it :)

If you are still using a Finder, simply replace it with a ExtractiveQAPipeline:

from haystack.pipeline import ExtractiveQAPipeline
pipe = ExtractiveQAPipeline(reader=..., retriever=...)
pipe.run(query="Some question")

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code
• Added tests
• Updated documentation

[lalitpagaria]

Please add small document or paragraph about way to migrate from Finder to ExtractiveQAPipeline in documentation.
It would be great to have this as part of version history on documentation page.

User have to -

1. Change import
2. Class from finder to pipeline
3. final output from answers to documents

[lalitpagaria]

LGTM :)