MINOR: pattern: make the delete and prune functions more generic

MINOR: pattern: make the delete and prune functions more generic

Now we have a single prune() function to act on an expression, and one
delete function for the lists and one for the trees. The presence of a
pointer in the lists is enough to warrant a free, and we rely on the
PAT_SF_REGFREE flag to decide whether to free using free() or regfree().