[keyserver] Fix siwe_nonces table creation

Summary:
Very stupid issue I noticed when creating a new database. The stray comma at the end causes the following:

Test Plan:

1. Create a new DB
2. Run keyserver
3. Ensure that the script runs successfully.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D5943