Remove a redundant chip erase from run-tests.go.  The '-e chip' should
tell pyocd to perform a chip erase.  There seem to be some combinations
of daplink and pyocd that if an invalid image is loaded into the device,
it can no longer be flashed by pyocd.  Performing the chip erase as a
separate command easily gets it into this state.  It can be recovered by
using the DAP filesystem to write the image.

Signed-off-by: David Brown <david.brown@linaro.org>