Remove a stale comment on MidiManagerUsb

Bug: None
Change-Id: I3daee8d38c9793a75e3506d8c16588e83506cc66
Reviewed-on: https://chromium-review.googlesource.com/1244181Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594623}

media/midi/midi_manager_usb.h

@@ -76,8 +76,6 @@ class USB_MIDI_EXPORT MidiManagerUsb : public MidiManager,
   // result.
   // When this factory is destroyed during the operation, the operation
   // will be canceled silently (i.e. |callback| will not be called).
-  // The function is public just for unit tests. Do not call this function
-  // outside code for testing.
   void Initialize(Callback callback);
 
   void OnEnumerateDevicesDone(bool result, UsbMidiDevice::Devices* devices);