Remove FlowFixMe's from TypeScript

Summary: This diff was a joy to draft.

Reviewed By: jknoxville

Differential Revision: D17344676

fbshipit-source-id: 0694d0393fa243edef61408ecfee92912fbbfa32

src/MenuBar.tsx

@@ -98,7 +98,6 @@ export function setupMenuBar(
         menuItem => menuItem.label === topLevelMenu,
       );
       if (menu) {
-        // $FlowFixMe submenu is missing in electron API spec
         const menuItem = menu.submenu.items.find(
           menuItem => menuItem.label === label,
         );