Remove special support for celery from sitecustomize

[owais]

Description

This is not required anymore after merging open-telemetry/opentelemetry-python#2242

Context: open-telemetry/opentelemetry-python#2266

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Existing tests

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[blumamir]

Hi @owais ,
I am trying to investigate an issue with celery spans not being present on the worker process in python 3.6. Came across this PR which removed the support for it in sitecustomize.py in favor of register_at_fork.

Since this function is only introduced in 3.7, I wonder if there is any recommended initialization pattern with auto-instrument that will make it work with python 3.6