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 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.63%. Comparing base (0fc8a49) to head (900dee6).

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    79.63%   79.63%           
========================================
  Files         1733     1733           
  Lines       197686   197683    -3     
  Branches     18226    18167   -59     
========================================
- Hits        157418   157417    -1     
+ Misses       40268    40266    -2     

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