[MINOR UPDATE] remove unused mysql.version property

[pjfanning]

What's changed?

This property in your pom file appears to be unused.

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Add or update API

• I have added the necessary e2e tests and all cases have passed.

[Calvin979]

It is not an unused property

[pjfanning]

Thanks @Calvin979, the search in GitHub didn't show that. I will close this and raise an issue instead.
The issue will be about why is Hertzbeat using such an out of date version of MySQL Connector? MySQL Connector also has a Category X license and ASF bans the use of Category X licenses.