Bug 1710018: Remove the shutdown assertion from pref_SetPref to just rely on the NS_ERROR_ILLEGAL_DURING_SHUTDOWN failure return. r=dom-worker-reviewers,jesup

Differential Revision: https://phabricator.services.mozilla.com/D137171

modules/libpref/Preferences.cpp

@@ -1586,9 +1586,6 @@ static nsresult pref_SetPref(const nsCString& aPrefName, PrefType aType,
   MOZ_ASSERT(NS_IsMainThread());
 
   if (AppShutdown::IsInOrBeyond(ShutdownPhase::XPCOMShutdownThreads)) {
-    MOZ_ASSERT(
-        false,
-        "!AppShutdown::IsInOrBeyond(ShutdownPhase::XPCOMShutdownThreads)");
     return NS_ERROR_ILLEGAL_DURING_SHUTDOWN;
   }