test-set: drop left-over valgrind check

In b01f31954f mempool_use_allowed
was dropped, but apparently it was forgotten here.

src/test/test-set.c

@@ -5,8 +5,6 @@
 #include "strv.h"
 #include "tests.h"
 
-const bool mempool_use_allowed = VALGRIND;
-
 TEST(set_steal_first) {
         _cleanup_set_free_ Set *m = NULL;
         int seen[3] = {};