Remove set but not used variables

[mgudemann]

clang 19.1.17 warns about set but not used variables and stops compilation because of -Werror

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.37%. Comparing base (41e9f3d) to head (0e7b4ac).
Report is 10 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/solvers/flattening/boolbv_overflow.cpp	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8595      +/-   ##
===========================================
- Coverage    80.38%   80.37%   -0.01%     
===========================================
  Files         1686     1686              
  Lines       206770   206767       -3     
  Branches        85       85              
===========================================
- Hits        166207   166198       -9     
- Misses       40563    40569       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[tautschnig]

@mgudemann Please rebase as the set of required CI checks has been adjusted.