Unused optional elements

The scripts parsing this file don't care if an element is optional or
not, so why bother.

Change-Id: I0860c9209347d6fab83e1faf2867db4de94216ac
DOCX export: handle date SDT on paragraphs

Date SDT's are normally imported as form controls, while most other SDT
types are just custom properties on regular text portions or paragraphs.

However, given that form controls are not supported in headers/footers,
in that case even date SDT's are just custom properties. So support such
properties on paragraphs in the exporter to properly roundtrip date
SDT's in headers/footers.

Change-Id: I19eb73a3673e387a7b8780756ce7426a1851e796