Downgrade bug 7164 warning to INFO

The 0.2.5.x warning is the one that might help us track this down; the
warnings in stable are just annoying users over and over and over.

changes/bug7164_downgrade

+  o Minor bugfixes:
+    - Downgrade the warning severity for the the "md was still referenced 1
+      node(s)" warning. Tor 0.2.5.4-alpha has better code for trying to
+      diagnose this bug, and the current warning in earlier versions of
+      tor achieves nothing useful. Addresses warning from bug 7164.
+

src/or/microdesc.c

@@ -561,7 +561,7 @@ microdesc_free(microdesc_t *md)
         }
       });
     if (found) {
-      log_warn(LD_BUG, "microdesc_free() called, but md was still referenced "
+      log_info(LD_BUG, "microdesc_free() called, but md was still referenced "
                "%d node(s); held_by_nodes == %u", found, md->held_by_nodes);
     } else {
       log_warn(LD_BUG, "microdesc_free() called with held_by_nodes set to %u, "