Revert "cleanup obsolete sps-pps-idr field trial"

This reverts commit 4f79b1d2e5f8754237657904dd1e6aa766fb6282.

Reason for revert: Still used in one project. I'll make a fix for that and then reland this.

Original change's description:
> cleanup obsolete sps-pps-idr field trial
>
> which has been superseeded by the equivalent nonstandard sdp fmtp
>   sps-pps-idr-in-keyframe
> parameter.
>
> Bug: webrtc:11769
> Change-Id: I02667a165dd3f86b4685530c43f19531ec654737
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/271121
> Commit-Queue: Philipp Hancke <phancke@microsoft.com>
> Reviewed-by: Erik Språng <sprang@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#37839}

Bug: webrtc:11769
Change-Id: I11e097e00813b7b232e01b236510cbf1b2850843
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/272560
Reviewed-by: Philipp Hancke <phancke@microsoft.com>
Bot-Commit: rubber-stamper@appspot.gserviceaccount.com <rubber-stamper@appspot.gserviceaccount.com>
Commit-Queue: Philipp Hancke <phancke@microsoft.com>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#37862}