Simplify undici instrumentation and remove feature flags

[bizob2828]

Description

We have feature flags gating undici. It's also making instrumenting global fetch more difficult because it'd require users to enable the undici feature flag as fetch is just undici under the hood.

Acceptance Criteria

As a New Relic Node.js customer I want the ability to track undici requests as external calls.

[workato-integration]

https://issues.newrelic.com/browse/NR-156456

[bizob2828]

Turns out the patch solves some issues and creates others around the span relationship of concurrent undici requests. If keep alive in enabled they're broken but you can disable the undici_async_tracking feature flag. We will keep this flag and just remove the undici feature flag.