metadata: Fix cannot remove optional metadata_type

Hi, when I add an optional metadata_type into a document, then I need to remove it, the application complaint that metadata_type is flagged as required.

Step to reproduce:

1. Add an optional metadata_type into a document_type
2. add the metadata_type into a specific document
3. remove that metadata_type by ticking the checkbox and click remove

Images attached

The last metadata_type is optional

Tick the checkbox and click remove