Example.dtx advertises assignment key for ECON, LAW, and SCI, but only LAW includes `\thesis@blocks@assignment`

Created by: Witiko

A student of ECON is unable to include an assignment in their thesis. It turns out that example.dtx advertises the assignment key for ECON, LAW, and SCI, but only LAW (for which we don't advertise it) includes \thesis@blocks@assignment in the back matter:

https://github.com/Witiko/fithesis/blob/3c878cfaeffc0b0498a6d38c34214fa179a6d338/style/mu/law.dtx#L164-L166

Git blame suggests that https://github.com/Witiko/fithesis/commit/4c50c07 deleted \thesis@blocks@assignment from econ.dtx. How should we handle this, @xvrabcov?