fixes #216, remove unnecessary call to startIntersectionObserver

[dbashford]

Closes #216

Changes proposed in this pull request

The in-viewport service has a startIntersectionObserver function which creates an instance of observerAdmin. The in-viewport mixin has a call to startIntersectionObserver that does not check to see if observerAdmin already exists. This resulted in the single instance of observerAdmin inside the service being replaced every time a component using the mixin was created. observerAdmin has WeakMap "registry"s of element references that are useful to have access to when debugging issues using this addon, and the instance changing made that impossible to leverage.

The mixin's call to startIntersectionObserver proves to be unnecessary as it is immediately called inside watchElement. That call IS protected so that observerAdmin is only ever created once.

This PR removes the mixin's call to startIntersectionObserver.

Thanks, all!

[snewcomer]

This is great! Thank you 💯 for taking the time to dig through!