This stops destructing the stencils held by ScriptPreloader on release build,
to avoid the shutdown hang caused by thrashing.
Also:
* Move ScriptPreloader::Cleanup from xpcom-shutdown notification to ScriptPreloader destructor, given there's no reason for having 2 steps
* Replace ClearOnShutdown call with explicit function call for the singleton deletion, for simplicity
* Remove now-unused PostJSShutDown phase
Differential Revision: https://phabricator.services.mozilla.com/D130839
