As stated in the documentation
here, there is a limitation when creating a GP service from the Export Web Map tool:
Caution:
Due to a known limitation with the Export Web Map tool, you cannot filter the parameter choices available to end users.This is a request for this limitation to be overcome and allow users to specify the layout templates and image formats at the time of publishing. The most important reason for this is to allow users to remove the ‘MAP_ONLY’ template as the user has no control over the content in the template. This means that an end user who chooses this template will be able to generate a print without elements like a copyright which can put the publisher in breach of any licence agreement with the vendor of the data in the print.This issue will affect customers using ArcGIS Online the most. This is because the GP task cannot be modified so to remove the ‘MAP_ONLY’ template and image formats, customisation is required at the client side. The customisation will also differ depending on the client/application used. 