Remove unused logic variables

[gamburgm]

Don't freshen logic variables that aren't referenced. For example, transform:

(fresh (a b c d)
    (== a '())
    (conso c a '(1 2 3)))

into:

(fresh (a c)
    (== a '())
    (conso c a '(1 2 3)))

This pass does not eliminate freshes if they're empty, as that's not semantics preserving.