Remove ember-maybe-in-element dependency

[maxfierke]

ember-maybe-in-element was used as a quick drop-in replacement for ember-wormhole when the latter became problematic for more recent Ember versions. However, version 2.0.0+ of ember-maybe-in-element dropped support for Ember < 3.13, which this addon still supports, and uses a Glimmer-component based approach, rather than an AST transform to equivalent in-element calls. As raised in #406, pinning to 1.0.0 is problematic for folks using newer versions of ember-maybe-in-element either directly or via popular addons like ember-basic-dropdown.

The most compatible solution, rather than directly importing the AST transform from version 1.0, is to just use in-element directly. This results in a small amount of duplication, but I think it's a worthwhile trade-off (duplication that happened during build time anyway, via the ember-maybe-in-element AST transform)