nixos/mattermost: remove fallback charset for MySQL

This was causing issues on newer versions of MariaDB (breaking
NixOS tests) like:

```
Error 1064 (42000): You have an error in your SQL syntax;
check the manual that corresponds to your MariaDB server version
for the right syntax to use near '%2Cutf8' at line 1
```

Since this is simply a fallback character set and all supported versions
of MariaDB support utf8mb4, delete the fallback.

This change should be fully compatible with existing deployments.

Changed files
+2 -2
nixos
modules
services
web-apps
+2 -2
nixos/modules/services/web-apps/mattermost.nix
···
}
else if cfg.database.driver == "mysql" then
{
-
charset = "utf8mb4,utf8";
+
charset = "utf8mb4";
writeTimeout = "60s";
readTimeout = "60s";
}
···
}
else if config.mattermost.database.driver == "mysql" then
{
-
charset = "utf8mb4,utf8";
+
charset = "utf8mb4";
writeTimeout = "60s";
readTimeout = "60s";
}