Remove iterator typedefs for USB descriptor types.

C++11 range-based for loops make it a lot easier to iterate over an STL
container without incredibly long type names. The last users of these
typedefs have been updated and so they are no longer necessary.

BUG=

Review URL: https://codereview.chromium.org/869063004

Cr-Commit-Position: refs/heads/master@{#312949}

device/usb/usb_descriptors.h

@@ -39,8 +39,6 @@ struct UsbEndpointDescriptor {
   UsbEndpointDescriptor();
   ~UsbEndpointDescriptor();
 
-  typedef std::vector<UsbEndpointDescriptor>::const_iterator Iterator;
-
   uint8_t address;
   UsbEndpointDirection direction;
   uint16_t maximum_packet_size;
@@ -55,8 +53,6 @@ struct UsbInterfaceDescriptor {
   UsbInterfaceDescriptor();
   ~UsbInterfaceDescriptor();
 
-  typedef std::vector<UsbInterfaceDescriptor>::const_iterator Iterator;
-
   uint8_t interface_number;
   uint8_t alternate_setting;
   uint8_t interface_class;