Revert "Prompt API Multimodal: Reimplement prototype ODMS Session usage"

This reverts commit 76a6c3614ee9def2abd886053e9a02729d227e6a.

Reason for revert: This caused tests failure https://ci.chromium.org/ui/p/chromium/builders/ci/Linux%20UBSan%20Tests/4078/overview

Original change's description:
> Prompt API Multimodal: Reimplement prototype ODMS Session usage
>
> crrev.com/c/6246232 used code pending removal in crrev.com/c/6253784
> Move the short-term multimodal prototype impl to supported patterns.
> (note token counts are incorrect when using this experimental flag)
>
> Bug: 385173789, 385173368
> Change-Id: I26ab60344c170ceeeed2e6ba3c23ff852f419694
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6303585
> Commit-Queue: Mike Wasserman <msw@chromium.org>
> Reviewed-by: Clark DuVall <cduvall@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1425301}

Bug: 385173789, 385173368
Change-Id: If25d8b2aba99ce583dda80292e6c2ab002b0ee83
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6307742
Commit-Queue: Shunya Shishido <sisidovski@chromium.org>
Owners-Override: Shunya Shishido <sisidovski@chromium.org>
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1425494}