Simplify helper agent unloading

[jsumners-nr]

Once the next version of (post 11.14.0) newrelic is released we can simplify the following snippet to only use the InstrumentationTracker:

node-test-utilities/lib/agent.js

Lines 160 to 167 in 3a32b2b

	if (
	Object.prototype.toString.call(shimmer.registeredInstrumentations) ===
	'[object InstrumentationTracker]'
	) {
	shimmer.registeredInstrumentations = new InstrumentationTracker()
	} else {
	shimmer.registeredInstrumentations = Object.create(null)
	}

[workato-integration]

https://new-relic.atlassian.net/browse/NR-255889