Delete racket/exists paragraph (#2873)

* Delete racket/exists paragraph

* Remove @ctc-link for exists-gotcha

pkgs/racket-doc/scribblings/guide/contracts/exists.scrbl

@@ -56,4 +56,3 @@ To ensure that the queue representation is abstract, we can use @racket[#:∃] i
 Now, if clients of the data structure try to use @racket[car] and @racket[cdr], they
 receive an error, rather than mucking about with the internals of the queues.
 
-See also @ctc-link["exists-gotcha"].

pkgs/racket-doc/scribblings/guide/contracts/gotchas.scrbl

@@ -108,27 +108,6 @@ Moral: if two values with contracts should interact,
        put them in separate modules with contracts at
        the module boundary or use @racket[#:freevar].
 
-@ctc-section[#:tag "exists-gotcha"]{Exists Contracts and Predicates}
-
-Much like the @racket[eq?] example above, @racket[#:∃] contracts
-can change the behavior of a program.
-
-Specifically,
-the @racket[null?] predicate (and many other predicates) return @racket[#f]
-for @racket[#:∃] contracts, and changing one of those contracts to @racket[any/c]
-means that @racket[null?] might now return @racket[#t] instead, resulting in
-arbitrarily different behavior depending on how this boolean might flow around
-in the program.
-
-@defmodulelang[racket/exists]
-
-To work around the above problem, the 
-@racketmodname[racket/exists] library behaves just like @racketmodname[racket],
-but predicates signal errors when given @racket[#:∃] contracts.
-
-Moral: Do not use predicates on @racket[#:∃] contracts, but if you're not sure, use
-@racketmodname[racket/exists] to be safe.
-
 @ctc-section{Defining Recursive Contracts}
 
 When defining a self-referential contract, it is natural to use