Retrieve no_proxy directly from the Datadog Agent's configuration

[nmuesch]

What does this PR do?

Remove the check for the no_proxy environment variables in get_instance_proxy. The logic should be done by the Datadog Agent and the https, http, and no_proxy options should all be available in the agents config to retrieve by the time the integrations request it.

Motivation

User had a no_proxy setup in the datadog.yaml file, however this wasn't being propogated to the integrations that use get_instance_proxy and instead these checks were using the proxy directly.

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

Additional Notes

Noticed some commented out pdbs in the network check, so removed those for cleanliness

[zippolyte]

LGTM

[ofek]

This too https://github.com/DataDog/integrations-core/blob/master/datadog_checks_base/datadog_checks/utils/proxy.py#L23

edit: forget this for now