Remove extraneous USBCON defines for AT90USB boards

USBCON is definied by serial.h pullikng in MarlinConfig.h which in turn pulls in Arduino.h.  Defining in later includes has shown to cause compile issues so removing all extraneous calls for and potentially easier future linker troubleshooting.
more USBCON unification

Marlin/pins_5DPRINT.h

@@ -74,7 +74,6 @@
 #define DEFAULT_MACHINE_NAME "Makibox"
 #define BOARD_NAME           "5DPrint D8"
 
-#define USBCON 1286  // Disable MarlinSerial etc.
 #define LARGE_FLASH        true
 
 //

Marlin/pins_BRAINWAVE.h

@@ -73,8 +73,6 @@
 
 #define BOARD_NAME         "Brainwave"
 
-#define USBCON 646  // Disable MarlinSerial etc.
-
 //
 // Limit Switches
 //

Marlin/pins_BRAINWAVE_PRO.h

@@ -80,7 +80,6 @@
 
 #define BOARD_NAME         "Brainwave Pro"
 
-#define USBCON 1286  // Disable MarlinSerial etc.
 #define LARGE_FLASH        true
 
 //

Marlin/pins_PRINTRBOARD.h

@@ -67,7 +67,6 @@
 
 #define BOARD_NAME         "Printrboard"
 
-#define USBCON 1286  // Disable MarlinSerial etc.
 #define LARGE_FLASH        true
 
 // Disable JTAG pins so they can be used for the Extrudrboard

Marlin/pins_SAV_MKI.h

@@ -69,7 +69,6 @@
 #define DEFAULT_SOURCE_CODE_URL "https://github.com/fmalpartida/Marlin/tree/SAV-MkI-config"
 #define BOARD_NAME              "SAV MkI"
 
-#define USBCON 1286  // Disable MarlinSerial etc.
 #define LARGE_FLASH        true
 
 //

Marlin/pins_TEENSY2.h

@@ -112,7 +112,6 @@
 
 #define BOARD_NAME         "Teensy++2.0"
 
-#define USBCON 1286  // Disable MarlinSerial etc.
 #define LARGE_FLASH        true
 
 //

Marlin/pins_TEENSYLU.h

@@ -79,7 +79,6 @@
 
 #define BOARD_NAME         "Teensylu"
 
-#define USBCON 1286  // Disable MarlinSerial etc.
 #define LARGE_FLASH        true