jbmc, janalyzer: Remove unnecessary dynamic_cast

`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.

jbmc/src/janalyzer/janalyzer_parse_options.cpp

@@ -401,13 +401,13 @@ int janalyzer_parse_optionst::doit()
   log.status() << "Generating GOTO Program" << messaget::eom;
   lazy_goto_model.load_all_functions();
 
-  std::unique_ptr<abstract_goto_modelt> goto_model_ptr =
+  std::unique_ptr<goto_modelt> goto_model_ptr =
     lazy_goto_modelt::process_whole_model_and_freeze(
       std::move(lazy_goto_model));
   if(goto_model_ptr == nullptr)
     return CPROVER_EXIT_INTERNAL_ERROR;
 
-  goto_modelt &goto_model = dynamic_cast<goto_modelt &>(*goto_model_ptr);
+  goto_modelt &goto_model = *goto_model_ptr;
 
   // show it?
   if(cmdline.isset("show-symbol-table"))

jbmc/src/jbmc/jbmc_parse_options.cpp

@@ -628,12 +628,16 @@ int jbmc_parse_optionst::get_goto_program(
 
     // Move the model out of the local lazy_goto_model
     // and into the caller's goto_model
-    goto_model_ptr = lazy_goto_modelt::process_whole_model_and_freeze(
-      std::move(lazy_goto_model));
-    if(goto_model_ptr == nullptr)
+    auto final_goto_model_ptr =
+      lazy_goto_modelt::process_whole_model_and_freeze(
+        std::move(lazy_goto_model));
+    if(final_goto_model_ptr == nullptr)
       return CPROVER_EXIT_INTERNAL_ERROR;
 
-    goto_modelt &goto_model = dynamic_cast<goto_modelt &>(*goto_model_ptr);
+    goto_modelt &goto_model = *final_goto_model_ptr;
+    goto_model_ptr =
+      std::unique_ptr<abstract_goto_modelt>(final_goto_model_ptr.get());
+    final_goto_model_ptr.release();
 
     if(cmdline.isset("validate-goto-model"))
     {