ScriptServiceProxy is a thin wrapper around the ScriptService which does
a runAs the xpack user when compiling. But script services know nothing
about xpack users, so this has no real effect. I believe this is a
remnant of when we had indexed scripts, where the compilation may have
done a get on the scripts index.
This change removes the ScriptServiceProxy. It also renames Script in
watcher to WatcherScript, to remove confusion between elasticsearch's
Script and watchers Script.
Original commit: elastic/x-pack-elasticsearch@4e2fdbc518
91441bbd2a