Similar to StartOffset:
1. There was a Windows non-cached RemoteAccessible implementation, but it was never actually called, so I removed it.
2. The sync IPDL RemoteAccessible implementation previously provided a boolean indicating success.
I removed this because the LocalAccessible implementation doesn't have this and it doesn't seem like remote is special in this respect.
Differential Revision: https://phabricator.services.mozilla.com/D138243