Fix SQL in cleaning up old notifications

[mecab]

Subtracting numbers from datetime causes wrong value, or raises error depending on MySQL version/configurations. It leads deleting notification wrongly or failed to do so.

This PR solves above issue by using INTERVAL to make appropriate value for date calculation.

Below confimed in MySQL8.0

mysql> select NOW() \G
*************************** 1. row ***************************
NOW(): 2018-01-01 13:25:56

mysql> select NOW() - 7 * 24 * 60 * 60\G
*************************** 1. row ***************************
NOW() - 7 * 24 * 60 * 60: 20180100527811

The result 20180100527811 is wrong. It is 20180101132611(string transformation by YYYYMMDDHHmmSS) - 7*24*60*60

mysql> select NOW() - INTERVAL 7 DAY\G
*************************** 1. row ***************************
NOW() - INTERVAL 7 DAY: 2017-12-25 13:28:18

I tried in MySQL 5.7 and had same result. And it ends up with error when this calculation used in WHERE clause.