Remove 'setvbuf' in string port redirect test.

This was only relevant to an older implementation of redirects, and
does not make much sense for us now.

* tests/unit/shell.scm (Redirects here-document to standard input):
Remove call to 'setvbuf'.

tests/unit/shell.scm

@@ -277,7 +277,6 @@
   "foo\n"
   (with-input-from-string "bar\n"
     (lambda ()
-      (setvbuf (current-input-port) 'none)
       (with-output-to-string
         (lambda ()
           (sh:with-redirects '((<< 0 "foo\n"))