fix: remove extra variable update

[Schlagonia]

Description

remove extra update of new_debt variable in _update_debt()

Fixes # (issue)

124

Checklist

• [x] I have run vyper and solidity linting
• [x] I have run the tests on my machine
• [x] I have followed commitlint guidelines
• [x] I have rebased my changes to the latest version of the main branch