Remove dynamic_cast from counterexample beautification code path

[tautschnig]

We know at solver-construction time whether we have one deriving from boolbvt.

• 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.
• n/a 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 12 lines in your changes missing coverage. Please review.

Project coverage is 77.94%. Comparing base (89a0470) to head (ede4a3a).
Report is 10 commits behind head on develop.

Files	Patch %	Lines
src/goto-checker/solver_factory.cpp	69.56%	7 Missing ⚠️
src/goto-checker/goto_symex_property_decider.cpp	0.00%	2 Missing ⚠️
src/goto-checker/multi_path_symex_checker.cpp	0.00%	1 Missing ⚠️
...-checker/single_loop_incremental_symex_checker.cpp	0.00%	1 Missing ⚠️
src/goto-checker/single_path_symex_checker.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8421   +/-   ##
========================================
  Coverage    77.93%   77.94%           
========================================
  Files         1726     1726           
  Lines       189657   189643   -14     
  Branches     18255    18231   -24     
========================================
+ Hits        147805   147810    +5     
+ Misses       41852    41833   -19     

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