Remove async api if it is not used by other clients downstream

I don't know if we are using it somewhere, but we should not break it without investigating where this is used.
I suggest we create a issue to track removing this from blueapi, But let this PR in so that we can have the latest changes without breaking anything.

_Originally posted by @ZohebShaikh in https://github.com/DiamondLightSource/blueapi/issues/1656#issuecomment-5582978063_

Start with the async API referenced in this issue and review issue #1656 for context. Investigate whether downstream clients use it before deciding the removal scope; the work is complete when usage is documented and a follow-up removal plan or issue is established without breaking dependent clients.

Written by the indexing model from the issue text.