pba-d-01-kw2x: remove duplicate MCPU

MCPU is already defined to `CPU_ARCH` which is `cortex-m4`.
Removing this line does not change the value:

    make --no-print-directory -C examples/hello-world/ BOARD=pba-d-01-kw2x \
        info-debug-variable-CPU_ARCH info-debug-variable-MCPU
    cortex-m4
    cortex-m4

boards/pba-d-01-kw2x/Makefile.include

@@ -6,8 +6,6 @@ export CPU = kinetis
 # cpu if needed.
 export CPU_MODEL ?= mkw21d256vha5
 
-export MCPU = cortex-m4
-
 # set default port depending on operating system
 PORT_LINUX ?= /dev/ttyACM0
 PORT_DARWIN ?= $(firstword $(sort $(wildcard /dev/tty.usbmodem*)))