fix(migrations): remove explicit BEGIN/COMMIT from migrations 0012-0015

applyMigration() already wraps each file in a db.Begin()/tx.Commit()
transaction. The inner BEGIN/COMMIT in these files caused the outer
transaction to commit early, leaving applyMigration trying to insert
into schema_version on an idle connection:
  pq: unexpected transaction status idle

Affected: 0012_guild_invites, 0014_return_guilds, 0015_tournament.

server/migrations/sql/0015_tournament.sql

@@ -1,5 +1,3 @@
-BEGIN;
-
 CREATE TABLE IF NOT EXISTS tournaments (
     id          SERIAL PRIMARY KEY,
     name        VARCHAR(64) NOT NULL,
@@ -44,5 +42,3 @@ CREATE TABLE IF NOT EXISTS tournament_results (
     stage_handle  INTEGER NOT NULL DEFAULT 0,
     submitted_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
 );
-
-COMMIT;