chore: Remove already tracked subscription counter

[joshua-spacetime]

We already track subscription requests through the rdb_num_txns counter.

Expected complexity level and risk

1

Testing

Describe any testing you've done, and any testing you'd like your reviewers to do,
so that you're confident that all the changes work as expected!

• Write a test you've completed here.
• Write a test you want a reviewer to do here, so they can check it off when they're satisfied.

[Centril]

Makes sense 👍