Fix category error when creating amendments

[bspekker]

Describe the bug
When the setting for creating new amendments are set to "Edit the whole motion text" or "Empty text field", users without motion manage permissions can't create amendments to motions with a category:

In general it is not necessary to set or change the category when creating an amendment, so the field can be removed from the view.

How to Reproduce

1. change the > settings > amendments > How to create new amendments to either "Edit the whole motion text" or "Empty text field"
2. give a user A permissions to create motions and amendments (and NOT motion manage)
3. create a motion X with a category
4. login as user A
5. create an amendment to motion X (and don't change the category) -> error:

Expected behavior

• the category field should be removed from the "new amendment" form
• the client should not send the category to the backend in the payload when creating an amendment