platform/exp5438_2520: fix Msp430TimerMicroMap.

formerly, the map had T0A1 allocated for the SFDCapture.  But this moved
to T0A3 (in HplCC2520C).  But the MicroMap didn't change to reflect.
Fix it.
platform/exp5438_2591: PlatformPins shouldn't touch gp5

The dual chip 2520/2591 implementation has gp5 wired directly and the
MCU doesn't have it.  So don't initialize the non-existent pin.
platforms/exp5438_25{20,91}: clean up, remove various kruft