The value isn't used; a 'dep tag is ony to establish
dependencies, and the relevant value is with a 'form or
'def tag. Avoiding the extra value saves another 5% in
cross-reference files.
This change saves a small amount of space in cross-reference files
and some space in loaded cross-reference information.
It also saves work converting strings to mutable on deserialize,
although the performance difference seems negligible.
