[vm, io] Remove dead Windows 7 code.

TEST=ci
Bug: #54509
Change-Id: I488f01e43faccf6f9f2dd58fcd55ca33a7f1f55d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/369860
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/bin/eventhandler_win.cc

@@ -148,15 +148,7 @@ void Handle::Close() {
     // If the handle uses synchronous I/O (e.g. stdin), cancel any pending
     // operation before closing the handle, so the read thread is not blocked.
     BOOL result = CancelIoEx(handle_, nullptr);
-// The Dart code 'stdin.listen(() {}).cancel()' causes this assert to be
-// triggered on Windows 7, but not on Windows 10.
-#if defined(DEBUG)
-    if (IsWindows10OrGreater()) {
-      ASSERT(result || (GetLastError() == ERROR_NOT_FOUND));
-    }
-#else
-    USE(result);
-#endif
+    ASSERT(result || (GetLastError() == ERROR_NOT_FOUND));
   }
   if (!IsClosing()) {
     // Close the socket and set the closing state. This close method can be