The enrollmentHelper was much more complicated than it needed to be. The
internal asynchrony that required awaiting an additional promise was fixed in
bug 1773583.
The returned cleanup function is no longer async, so unnecessary awaits have
been removed. This also applies to enrollWithFeatureConfig, as it is a wrapper
around enrollmentHelper.
Differential Revision: https://phabricator.services.mozilla.com/D212318
