Revert "WebNN: manage context provider impls"

This reverts commit 1d02bc6ca08e6059c7df5883cae8ca0ba34e7c92.

Reason for revert: causes issue https://issues.chromium.org/issues/347114487

Original change's description:
> WebNN: manage context provider impls
>
> Moves WebNNContextProviderImpl instances into a manager by converting
> them to become normal Mojo receivers. This gets us one step closer to
> interop by allowing provider access within the GPU service.
>
> Bug: 40278771
> Change-Id: I292849b68f456a3d8febe074d1f688f090b8d88b
> Cq-Include-Trybots: luci.chromium.try:win11-blink-rel
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5590076
> Reviewed-by: Austin Sullivan <asully@chromium.org>
> Commit-Queue: Bryan Bernhart <bryan.bernhart@intel.com>
> Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
> Cr-Commit-Position: refs/heads/main@{#1314115}

Bug: 40278771,347114487
Change-Id: I9b3b87befecc1025e010e0fa0327c33fdddb996d
Cq-Include-Trybots: luci.chromium.try:win11-blink-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5634869
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Rafael Cintron <rafael.cintron@microsoft.com>
Commit-Queue: ningxin hu <ningxin.hu@intel.com>
Cr-Commit-Position: refs/heads/main@{#1316264}