Bug 1845924, no need for DIAGNOSTIC_ASSERT, r=peterv

Differential Revision: https://phabricator.services.mozilla.com/D185040

docshell/base/BrowsingContext.cpp

@@ -3441,7 +3441,7 @@ ChildSHistory* BrowsingContext::GetChildSessionHistory() {
 void BrowsingContext::CreateChildSHistory() {
   MOZ_ASSERT(IsTop());
   MOZ_ASSERT(GetHasSessionHistory());
-  MOZ_DIAGNOSTIC_ASSERT(!mChildSessionHistory);
+  MOZ_ASSERT(!mChildSessionHistory);
 
   // Because session history is global in a browsing context tree, every process
   // that has access to a browsing context tree needs access to its session