Change par-mapper' to use fetch-result-of-defered-thunks

So the fetch-result-of-defered-thunk procedure can be removed.

guix-data-service/utils.scm

@@ -185,7 +185,7 @@
                (head (defer-to-thread-pool-channel
                        (lambda ()
                          (apply proc heads)))))
-           (cons (fetch-result-of-defered-thunk head) tail)))
+           (cons (fetch-result-of-defered-thunks head) tail)))
         (_
          '())))))