[HID] removed HIDDescriptor field

The field is now built on-the-fly on the stack and sent over USB.
This change increase Flash usage and decrease SRAM usage:

before: 6114 / 241
after:  6152 / 216 (removed HIDDescriptor field)

delta:   +38 / -25

SRAM is a much more scarse resource and this change free up to
about 10% of the "base" usage.

hardware/arduino/avr/libraries/HID/HID.cpp

@@ -26,7 +26,7 @@ HID_ HID;
 int HID_::getInterface(uint8_t* interfaceNum)
 {
 	*interfaceNum += 1; // uses 1
-	hidInterface = {
+	HIDDescriptor hidInterface = {
 		D_INTERFACE(interface(), 1, 3, 0, 0),
 		D_HIDREPORT(descriptorSize),
 		D_ENDPOINT(USB_ENDPOINT_IN(endpoint()), USB_ENDPOINT_TYPE_INTERRUPT, USB_EP_SIZE, 0x01)

hardware/arduino/avr/libraries/HID/HID.h

@@ -84,7 +84,6 @@ protected:
   bool setup(USBSetup& setup, uint8_t interfaceNum);
 
 private:
-  HIDDescriptor hidInterface;
   uint8_t epType[1];
 
   HIDDescriptorListNode* rootNode;