mysql schema: remove theme_id index pointing nowhere

schema/ttrss_schema_mysql.sql

@@ -52,8 +52,7 @@ create table ttrss_users (id integer primary key not null auto_increment,
 	created datetime default null,
 	twitter_oauth longtext default null,
 	otp_enabled boolean not null default false,
-	resetpass_token varchar(250) default null,
-	index (theme_id)) ENGINE=InnoDB DEFAULT CHARSET=UTF8;
+	resetpass_token varchar(250) default null) ENGINE=InnoDB DEFAULT CHARSET=UTF8;
 
 insert into ttrss_users (login,pwd_hash,access_level) values ('admin',
 	'SHA1:5baa61e4c9b93f3f0682250b6cf8331b7ee68fd8', 10);