tests: Remove stale reference to %derivation-cache.

* guix/tests.scm (call-with-external-store): Remove the stale reference to
%derivation-cache.  This was causing the tests to error out, since it was
removed from (guix packages) in 7d873f194c.

guix/tests.scm

@@ -141,10 +141,6 @@ no external store to talk to."
     (dynamic-wind
       (const #t)
       (lambda ()
-        ;; Since we're using a different store we must clear the
-        ;; package-derivation cache.
-        (hash-clear! (@@ (guix packages) %derivation-cache))
-
         (proc store))
       (lambda ()
         (when store