Follow-up to #41823.
Now that we always execute JS with callback, we can safely remove the
obsolete sync script handler.
- Rename `ExecuteAsyncScript` to `ExecuteScriptWithCallback`
- Remove `ExecuteScript` related logic.
- Rewrite async callback wrapper to simplify things.
Testing: I expect no change in behaviour. See if WPT tests agree with
it.
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Signed-off-by: Euclid Ye <yezhizhenjiakang@gmail.com>