Keyboard shortcuts: volume shortcuts should be local

`globalShortcut` will register the shortcut at the OS level, even when
the app is not focused.

Using `localShortcut` would work, but let's put it in the top menu
instead, where all the other shortcuts are.

main/shortcuts.js

@@ -18,8 +18,4 @@ function init () {
   // Electron does not support multiple accelerators for a single menu item, so this
   // is registered separately here.
   localShortcut.register('CmdOrCtrl+Shift+F', menu.toggleFullScreen)
-
-  // Control Volume
-  globalShortcut.register('CmdOrCtrl+Up', () => windows.main.send('dispatch', 'changeVolume', 0.1))
-  globalShortcut.register('CmdOrCtrl+Down', () => windows.main.send('dispatch', 'changeVolume', -0.1))
 }