board: arduino-due, mbed_lpc1768, udoo: remove redundant CPU_MODEL define

boards/arduino-due/Makefile.include

 # define the cpu used by the arduino due board
 export CPU = sam3x8e
-export CPU_MODEL = sam3x8e
 
 # define the default port depending on the host OS
 PORT_LINUX ?= /dev/ttyACM0

boards/mbed_lpc1768/Makefile.include

 # define the cpu used by the mbed_lpx1768 board
 export CPU = lpc1768
-export CPU_MODEL = lpc1768
-
 
 export FLASHER = $(RIOTBOARD)/$(BOARD)/dist/flash.sh
 export DEBUGGER =

boards/udoo/Makefile.include

 # define the cpu used by the udoo board
 export CPU = sam3x8e
-export CPU_MODEL = sam3x8e
 
 #define the flash-tool and default port depending on the host operating system
 OS := $(shell uname)