`event-handler`: Teleport "namespace" is still exposed as a configuration option

[jof]

The event-handler plugin still exposes Teleport namespace as a configuration option in the config file.

This confused me for a while, as this appears to be required in the RPC Client to Teleport, but is always set to "default" and doesn't really seem like a fully developed feature inside of Teleport.

This option should probably just be hard coded to "default" inside the plugin and not exposed as an option to end users.