Revert "Enable MojoChannelAssociatedSendUsesRunOrPostTask in testing config."

This reverts commit 4e241a955d24cf7ea2569d7f6b318815b81f8f43.

Reason for revert: No active experiment while we're investigating crashes.

Original change's description:
> Enable MojoChannelAssociatedSendUsesRunOrPostTask in testing config.
>
> This feature allows message to be sent on a channel-associated interface
> without posting to the sending process' IO thread.
>
> Bug: 1503967
> Change-Id: I62e76e39134b11c8059794649923859c7b98ee56
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5169427
> Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
> Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
> Cr-Commit-Position: refs/heads/main@{#1243845}

Bug: 1503967
Change-Id: I19095343577a97c43ae1841f25271a62dd377818
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5225361
Reviewed-by: Etienne Pierre-Doray <etiennep@chromium.org>
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Auto-Submit: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1252090}