Remove unused static ExecutionContxt/ScriptState getters based on realm

ExecutionContext's ForCurrentRealm() and ForReleveantRealm() are
entirely unused

ScriptState's ForRelevantRealm() variants are all unused except the
one that takes a v8::Local<v8::Object>.

This removes the last reference to V8CrossOriginCallbackInfo, so
delete that, too.

Change-Id: I34bc516ca07604ab987b82ccd2c783fc79c28dfb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4528137
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Auto-Submit: Nate Chapin <japhet@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1144311}