Remove deprecated observer method, OnIceCandidatesRemoved

The `OnIceCandidatesRemoved` observer method was previously deprecated
in favor of the more granular, IceCandidate based, `OnIceCandidateRemoved` method. As downstream users have migrated to the
new API, the deprecated method and its backward compatibility shims can
now be safely removed.

This change removes the `OnIceCandidatesRemoved` method from the
`PeerConnectionObserverInterface` and deletes the compatibility code
from the default `OnIceCandidateRemoved` implementation. The logic in
`PeerConnection::RemoveIceCandidates` is also simplified accordingly.

Bug: webrtc:42233526
Change-Id: I7b14ce4c2b387e7e75f99606474d73bb4a3df5f2
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/407280
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#45607}