Remove IAsyncTexlFunction4 and IAsyncTexlFunction5

[MikeStall]

These are used by implementatons from Fx.Json.

The challenge is that Fx.Json can't depend on Fx.Interpreter (because PAClient depends on Fx.Json).
That's a unique requirement - most everything else that wants to provide an interpreter implementation can safely depend on Fx.Interpreter. So we may need to special case something here. Perhaps a unique PowerFxJson contract that lives in core, is implemented by Fx.Json, and handled by Fx.Interpreter

[MikeStall]

As part of this, IAsyncTexlFunction5 should not get its own ServiceProvider.

[anderson-joyle]

A reminder that we will need to act on PFx.DV since IAsyncTexlFunction5 it is used also there.