[Feed] Part9 of remove web feed feature: Remove unread content observer and rss related code

This CL continues the deprecation of the Web Feed (Following feed)
feature by cleaning up orphaned infrastructure and dead code:

1. UnreadContentObserver: Removed `UnreadContentObserver`,
`JavaUnreadContentObserver`, and their associated JNI bridges. This
observer was exclusively instantiated and registered for
`StreamKind.FOLLOWING` to track whether the Web Feed had unread content
[1]. With Web Feed removed, this observer is never used.

2. RSS Link Fetching: Removed `RssLinksFetcher`,
`WebFeedPageInformationFetcher`, and the cross-process
`mojom::RssLinkReader` interface. This pipeline was built exclusively
for the Web Feeds system to extract RSS/Atom links from the renderer
process [2]. Because Web Feed was the sole consumer of this Mojo
interface, the entire fetching infrastructure and its associated
unit/browser tests are deleted.

3. Histogram: ContentSuggestions.Feed.WebFeed.GetRssLinksRendererTime is
removed

[1]
https://drive.google.com/file/d/1VBYqSmyaXK9Kj6qmLkGXDVPGGLm2oBQk/view?usp=sharing&resourcekey=0-y0MHR_JGdIJxGV_iIYYRRg
[2]
https://drive.google.com/file/d/1Ctsit3vsgjoHF22KGEoqMp0mbqooukQ9/view?usp=sharing&resourcekey=0-eSIkBHfiuwObsXBa3oiMYA

Cq-Include-Trybots: luci.chrome.try:android-internal-binary-size;luci.chrome.try:android-internal-rel;luci.chrome.try:android-internal-dbg
Bug: 407797637
Change-Id: Ie6cddb66801d62e481eceb1bb99dae48b1471fd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7795938
Reviewed-by: Giovanni Ortuno Urquidi <ortuno@chromium.org>
Reviewed-by: Patrick Noland <pnoland@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Reviewed-by: Xi Han <hanxi@chromium.org>
Commit-Queue: Yanling Hu <yanlinghu@google.com>
Cr-Commit-Position: refs/heads/main@{#1626918}