refactor[lang]: remove translated fields for constant nodes

[tserg]

What I did

Remove deprecated properties for AST nodes when using python versions <3.8, which were translated to the new properties for python versions >=3.8.

How I did it

How to verify it

Commit message

This commit removes the `.s` field from the AST for string-like
constants. These were deprecated in the Python AST in 3.8; we no longer
need to translate them for our AST.

Description for the changelog

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[charles-cooper]

lgtm. thanks!