Remove useless yield in template (#18)

Author: AdrienVannson

src/betterproto/templates/template.py.j2

@@ -197,9 +197,6 @@ class {{ service.py_name }}Base(ServiceBase):
         {% endif %}
 
         raise grpclib.GRPCError(grpclib.const.Status.UNIMPLEMENTED)
-        {% if method.server_streaming %}
-        yield {{ method.py_output_message_type }}()
-        {% endif %}
 
     {% endfor %}