Revert "[fuchsia] Launch web_instance.cm from context_provider.cm"

This reverts commit 5e3215ba.

Reason for revert: Appears to fail upstream E2E tests, due to various
capabilities not being available to the web_instances.

Original change's description:
> [fuchsia] Launch web_instance.cm from context_provider.cm
>
> In so doing:
>
> - Add support for CreateContextParams.service_directory in
>   WebInstanceHost.
> - Add support to WebInstanceHost for running within a process with
>   `--with-webui` on its command line to launch child instances in the
>   `web_engine_with_webui` package rather than the `web_engine`
>   package. This feature is used by `web_engine_shell` in test scenarios.
>
> Bug: 1280703
> Fuchsia-Binary-Size: Size increase is unavoidable.
> Change-Id: I08d65492c4c670e67cca88beb223d1d8c8eed76f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4194689


> Reviewed-by: Rohan Pavone <rohpavone@chromium.org>
> Reviewed-by: Wez <wez@chromium.org>
> Commit-Queue: Greg Thompson <grt@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1108989}

Bug: 1280703
Bug: b/270955577
Change-Id: Ifc61fc5ec781f5624551310f7a0b968c2d1ad348
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4294973


Reviewed-by: Rohan Pavone <rohpavone@chromium.org>
Auto-Submit: Wez <wez@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1111658}