Replace FGS devices with PMAC flyer when ready

We can simplify our x-ray centring plan once https://github.com/bluesky/ophyd-async/issues/427 is ready. We should think about using this instead of our FGS devices once it is ready. Some advantages are:
- We can use the same PV's for zebra and panda scans
- We don't have to worry about motion scripts + the motion will be much easier to modify
- At the top level XRC plan, instead of choosing a FGS device, motors, and params, we instead just configure the PMAC in a different way. This will simplify the `BeamlineSpecificFGSFeatures` part of the code

## Acceptance Criteria
- Specific criteria that will be used to judge if the issue is fixed