Remove backend from service.template

kubernetes has been the only possible backend value since gridengine was
removed last year [1].

[1]: https://wikitech.wikimedia.org/wiki/News/2024_Toolforge_Grid_Engine_deprecation

Author: lucaswerkmeister

{{cookiecutter.tool_identifier}}/service.template

@@ -1,5 +1,4 @@
 # Toolforge webservice template
 # Provide default arguments for `webservice` commands for this tool.
-backend: kubernetes
 type: python3.11
 health-check-path: /healthz