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/0012_guild_invites.sql

@@ -1,5 +1,3 @@
-BEGIN;
-
 -- Dedicated table for guild-initiated scout invitations, separate from
 -- player-initiated applications. This gives each invitation a real serial PK
 -- so the client's InvitationID field can map to an actual database row
@@ -19,5 +17,3 @@ FROM guild_applications
 WHERE application_type = 'invited';
 
 DELETE FROM guild_applications WHERE application_type = 'invited';
-
-COMMIT;