[nix] remove extra comma

Summary: Missed this before landing. When completely resetting your mariadb, the default db-config.json will failed to be parsed with this comma

Test Plan: successfully ran keyserver after removing comma

Reviewers: ashoat, ginsu, bartek, O1 Nix

Reviewed By: bartek, O1 Nix

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12040