Remove dynamic_cast from bv_dimacst

[tautschnig]

We can just hold on to the dimacs_cnft typed instance of propt that is used for constructing the object.

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• n/a 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.
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[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.02%. Comparing base (5ef19c8) to head (30ff419).

Files	Patch %	Lines
src/solvers/flattening/bv_dimacs.cpp	57.14%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8406      +/-   ##
===========================================
- Coverage    78.02%   78.02%   -0.01%     
===========================================
  Files         1726     1726              
  Lines       189442   189448       +6     
  Branches     18466    18263     -203     
===========================================
+ Hits        147808   147809       +1     
- Misses       41634    41639       +5     

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