Helm-Chart: make the MySQL-Secret optional

[gtudan]

Describe the bug
The seaweedfs helm chart uses a secret to deploy the filer database credentials for MySQL. We don't Redis in our setup, so we don't really need those credentials. We would prefer not to have them, as the defaults are hardcoded. Making them optional would allow us to remove them, without breaking the deployment.

System Setup

• Seaweedfs deployed via Helm
• Helm 3 on Kubernetes 1.27
• Helm chart 3.67
• Redis as filer persistence (or any non-mysql datastore)

Expected behavior
It is possible to remove the database secert without breaking the deployment.

[gtudan]

I didn't want to put this into the description in order not to sidetrack the ticket - but is there a specific reason to include the MySQL-Config in the chart, although it is not enabled? It would still be possible to configure MySQL through the extraEnvironmentVars / secretExtraEnvironmentVars (like all other persistence options).