Bug 1919435 - [1/1] Fix MinGW build r=win-reviewers,gstoll

... by removing a stray IDE-inserted nonstandard header.

Differential Revision: https://phabricator.services.mozilla.com/D222661

widget/windows/WinEventObserver.cpp

@@ -4,7 +4,6 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-#include <corecrt_memcpy_s.h>
 #include <windows.h>
 #include <winternl.h>
 #include <winuser.h>