Revert "[driver_runtime] Override libasync default function"

This reverts commit 5aa9dfe4b0f2bd92f5e44fb61deef55e9a84cd30.

Reason for revert: Does not work correctly in dso_runner, which is the main reason it was landed, and fix is not trivial.

Original change's description:
> [driver_runtime] Override libasync default function
>
> This makes it so that you can use the `async-default` library to get the
> current driver dispatcher, just like in regular components, by
> overriding that library's function for getting it to try to get it from
> the driver dispatcher first.
>
> In the process, this allows us to move `CurrentDispatcher` in the rust
> abstractions up to the `libasync` bindings library instead, which will
> make it easier to use in code that may or may not be running in a driver
> context.
>
> Fixed: 488394483
> Change-Id: I4f67f5848b6d3a5a4f76f017b082d7e0eda779c6
> Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1518418
> Commit-Queue: Megan Batty <mgnb@google.com>
> Reviewed-by: Suraj Malhotra <surajmalhotra@google.com>
> Fuchsia-Auto-Submit: Megan Batty <mgnb@google.com>

Bug: 488394483
Change-Id: I1c322f1f82d4579ed076102414c18b560f83bf98
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1537181
Reviewed-by: Suraj Malhotra <surajmalhotra@google.com>
Commit-Queue: Megan Batty <mgnb@google.com>
Fuchsia-Auto-Submit: Megan Batty <mgnb@google.com>
Exempt: RubberStamper 🤖 <android-build-ayeaye@system.gserviceaccount.com>