Clean up PendingBeaconAPI-related implementation

It was an experimental API only enabled for OT from M107-M115 [1].
Since then, it has been replaced by the fetchLater API [2], which is
currently running in OT [3].

[1]: https://chromestatus.com/feature/5690553554436096
[2]: https://github.com/WICG/pending-beacon/blob/main/docs/fetch-later-api.md
[3]: https://chromestatus.com/feature/4654499737632768

OBSOLETE_HISTOGRAM[PendingBeaconHost.BatchAction]=Removed entire API
OBSOLETE_HISTOGRAM[PendingBeaconHost.Action]=Removed entire API

Bug: b:40213606
Change-Id: I391bb3d38cdf11b40f6dc7cd755eded0923deb7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5367498


Reviewed-by: Sean Harrison <harrisonsean@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Commit-Queue: Ming-Ying Chung <mych@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1276738}