Refactor AnnotationAgentImpl removal code

We previously had a situation where AnnotationAgentImpl would ask its
owner to remove itself in its Remove method; which led to some extra
complexity when trying to remove multiple agents in
AnnotationAgentContainerImpl::RemoveAgentsOfType. This patch now forces
callers to call AnnotationAgentContainerImpl::RemoveAgent to remove
agents, instead of AnnotationAgentImpl::Remove.

Change-Id: If29e6adbb81557c8952d7a938e5f321f30925787
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6533918
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Adithya Srinivasan <adithyas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466492}