Move use_pool() to mempool.c and rename to mempool_enabled()

The only user is in hashmap.c, but it's a mempool thing.

src/basic/hashmap.c

@@ -6,7 +6,6 @@
 #include <string.h>
 
 #include "alloc-util.h"
-#include "env-util.h"
 #include "fileio.h"
 #include "hashmap.h"
 #include "macro.h"
@@ -767,24 +766,12 @@ static void reset_direct_storage(HashmapBase *h) {
         memset(p, DIB_RAW_INIT, sizeof(dib_raw_t) * hi->n_direct_buckets);
 }
 
-static bool use_pool(void) {
-        static int b = -1;
-
-        if (!is_main_thread())
-                return false;
-
-        if (b < 0)
-                b = getenv_bool("SYSTEMD_MEMPOOL") != 0;
-
-        return b;
-}
-
 static struct HashmapBase *hashmap_base_new(const struct hash_ops *hash_ops, enum HashmapType type HASHMAP_DEBUG_PARAMS) {
         HashmapBase *h;
         const struct hashmap_type_info *hi = &hashmap_type_info[type];
         bool up;
 
-        up = use_pool();
+        up = mempool_enabled();
 
         h = up ? mempool_alloc0_tile(hi->mempool) : malloc0(hi->head_size);
         if (!h)