Guide: remove a broken example

Using `syntax` to capture local binding information in the current
phase doesn't work with the set-of-scopes expander. Although the
example could be adjust to use `(quote-syntax car #:local)`, it
seems like too much detail at that point in the explanation.

pkgs/racket-doc/scribblings/guide/proc-macros.scrbl

@@ -67,13 +67,8 @@ to the same binding:
 (free-identifier=? #'car #'car)
 (require (only-in racket/base [car also-car]))
 (free-identifier=? #'car #'also-car)
-(free-identifier=? #'car (let ([car 8])
-                           #'car))
 ]
 
-The last example above, in particular, illustrates how syntax objects
-preserve lexical-context information.
-
 To see the lists, symbols, numbers, @|etc| within a syntax object, use
 @racket[syntax->datum]: