Remove environment variable configuration for usage tracking

[NishchayKarle]

Description

This PR removes the use of $PARSL_TRACKING env variable to setup usage tracking configuration. Parsl will now only take usage tracking configuration from the config

Changed Behavior

Anyone using only the env variable PARSL_TRACKING to enable usage tracking will need to set usage_tracking = True within the config to enable usage tracking.

Fixes

Fixes #3456

Type of change

• Code maintenance/cleanup
• Update documentation

[benclifford]

There are a couple other places where that variable is mentioned. I used git grep to find them:

$ git grep PARSL_TRACKING
docs/quickstart.rst:You can opt-in by setting ``PARSL_TRACKING=true`` in your environment or by 
docs/userguide/usage_tracking.rst:If you wish to opt in to usage reporting, set ``PARSL_TRACKING=true`` in your environment or set ``usage_tracking=True`` in the configuration object (`parsl.config.Config`).

This PR should fix those references too