Remove unused chrome.inputMethodPrivate.onSettingsChanged API.

This private API is meant for use ONLY by CrOS 1P VK+IME
extension, which hasn't been using it for long if at all.

It's long been superseded by onInputMethodOptionsChanged
introduced @ https://crrev.com/c/2264089

Highly likely that it also stopped working upon the rewiring to
onInputMethodOptionsChanged @ https://crrev.com/c/2362491

Bug: b:444382744
Change-Id: I9c81dfd158e6f33d4acbfd6ac2c4d61205ff71fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7036771
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Reviewed-by: Darren Shen <shend@chromium.org>
Reviewed-by: Jing Wang <jiwan@chromium.org>
Commit-Queue: Bao-Duy Tran <tranbaoduy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1529322}
NOKEYCHECK=True
GitOrigin-RevId: 424a9251e6c998bde94d4c11eeb70e6cdcde2c44