Allow turning shadowing `let*` to `define` when the name isn't used

[jackfirth]

Saw this code today:

    (let* ([f (new frame% (label "Break DrRacket"))]
           [b (new button% 
                   (label "Break Main Thread")
                   (callback
                    (λ (x y)
                      (break-thread to-break)))
                   (parent f))]
           [b (new button% 
                   (label "Break All Threads")
                   (callback
                    (λ (x y)
                      ((dynamic-require 'drracket/private/key 'break-threads))))
                   (parent f))])
      (send f show #t))

Resyntax won't rewrite this let* to define because it sees that the b identifier is bound twice. However, the b identifier is unused, so a better rewrite would be to change it to this:

(define f (new frame% (label "Break DrRacket")))
(new button% 
     (label "Break Main Thread")
     (callback
      (λ (x y)
        (break-thread to-break)))
     (parent f))
(new button% 
     (label "Break All Threads")
     (callback
      (λ (x y)
        ((dynamic-require 'drracket/private/key 'break-threads))))
     (parent f))
(send f show #t)

The let* to define refactoring rules should consider these cases.