secureboot.py: skip unnecessary cleaning
secureboot.py: skip unnecessary cleaning

Whatever the problem was, it no longer seems to be necessary, so we
can skip the time-consuming clean + full rebuild (5:04 min instead of
6:55min here).

Signed-off-by: Patrick Ohly <0fc9d825c622b2a5ac5160289c29f03654b10648@intel.com>

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