remove sfi links from the Reference

In consultation with Ryan. We'd prefer to have versions of all
useful things in a `racket/...' library that is consistent as
possible with Racket forms and conventions.

collects/scribblings/reference/pairs.scrbl

@@ -1028,8 +1028,6 @@ the result of @racket[proc]. Signals an error on an empty list.
 (range 10 15 1.5)
 ]}
 
-You can find additional list functions in @racketmodname[srfi/1].
-
 
 @close-eval[list-eval]
 

collects/scribblings/reference/strings.scrbl

@@ -432,6 +432,5 @@ true, otherwise they get normalized too.
  (string-normalize-spaces "  foo bar  baz \r\n\t")
 ]}
 
-You can find additional string functions in @racketmodname[srfi/13].
 
 @close-eval[string-eval]

collects/scribblings/reference/time.scrbl

@@ -220,6 +220,3 @@ calculations.}
 
 Converts a Julian number (Scalinger's off-by-one version) into a
 string.}
-
-You can find additional date and time functions in
-@racketmodname[srfi/19].