Current logic does not need to access inlays by id in O(1), future
dynamic hints would need to know which hint they hover at, but that will
be done using binary search over the position's anchor we hover on;
nothing else seems to need this HashMap in the near future.

Because of that removal, no need to store `InlayId` apart from the
`Inlay`, hence remove the `InlayProperties` struct entirely.
This allows to eliminate a few generics along the way.

Release Notes:

- N/A