Bug 1720314 - Part 1: Remove HTMLFormElement::mSubmittingRequest; r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D119776

dom/html/HTMLFormElement.cpp

@@ -1827,8 +1827,6 @@ int32_t HTMLFormElement::Length() { return mControls->Length(); }
 
 void HTMLFormElement::ForgetCurrentSubmission() {
   mNotifiedObservers = false;
-  mSubmittingRequest = nullptr;
-
   mTargetContext = nullptr;
   mCurrentLoadId = Nothing();
 }