Properly remove MUT_SLOW_HEALING from the given bad mutations.

give_bad_mutation doesn't heed the probabilities in mutation_defs, so MUT_SLOW_HEALING could be obtained but not cured.

This is a quick fix; give_bad_mutation should reallly be integrated with mutation_defs at some point.

Created by  Robert Vollmert  on October 7, 2009
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