fix(document): ensure post('deleteOne') hooks are called when calling save() after subdoc.deleteOne()

[vkarpov15]

Fix #9885

Summary

It looks like when we removed remove(), we didn't make it so that post('deleteOne') hooks fire when you save() a deleted doc. This PR makes Mongoose consistently trigger post('deleteOne') middleware on subdocs when you call subdoc.deleteOne(); await topLevelDoc.save();, which is what used to happen when you did subdoc.remove(); await topLevelDoc.save();

Examples

[vkarpov15]

I also refactored the subdoc post save hook to use async/await. Looks to be about 7% faster on the mapOfSubdocs benchmark on my local, so this refactor may actually be slightly faster.

We will do some work to add promise support for kareem, to avoid the need to explicitly instantiate new Promise()