jbmc, janalyzer: Remove unnecessary dynamic_cast

[tautschnig]

lazy_goto_modelt::process_whole_model_and_freeze returns a unique pointer to goto_modelt, which these call sites unnecessarily generalised to abstract_goto_modelt (just to then dynamic_cast it to goto_modelt). Fix the local pointer types and remove the now no longer necessary cast.

• Each commit message has a non-empty body, explaining why the change was made.
• 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.
• 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 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 77.92%. Comparing base (3877e0f) to head (3fef02a).
Report is 39 commits behind head on develop.

Files with missing lines	Patch %	Lines
jbmc/src/jbmc/jbmc_parse_options.cpp	87.50%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8418      +/-   ##
===========================================
- Coverage    77.94%   77.92%   -0.02%     
===========================================
  Files         1726     1726              
  Lines       189621   189661      +40     
  Branches     18247    18247              
===========================================
+ Hits        147792   147797       +5     
- Misses       41829    41864      +35     

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