Remove field masks that are not allowed on update

[lulf]

Follows the guidelines as outlined by https://www.thethingsindustries.com/docs/reference/api/field-mask/#put-requests

This part does not add any validation hook, but maybe the simplest course of action is to create a separate issue for that.

Edit: I've also not been able to test this yet.

[ctron]

Workaround for #298