fix: remove reckless assertion (#15)

Author: cnwangjie

lib/mysql.js

@@ -1,6 +1,5 @@
 'use strict';
 
-const assert = require('assert');
 const rds = require('ali-rds');
 
 let count = 0;
@@ -10,9 +9,6 @@ module.exports = app => {
 };
 
 function createOneClient(config, app) {
-  assert(config.host && config.port && config.user && config.database,
-    `[egg-mysql] 'host: ${config.host}', 'port: ${config.port}', 'user: ${config.user}', 'database: ${config.database}' are required on config`);
-
   app.coreLogger.info('[egg-mysql] connecting %s@%s:%s/%s',
     config.user, config.host, config.port, config.database);
   const client = rds(config);