Remove unused defined_names artifacts and simplify fingerprint_set::contains (#9702)

Cleans up dead code left by the "remove side definitions" refactoring
(a0a3047).

- **`smt_model_checker.cpp`** — Remove `defined_names dn(m)` variable
that was declared but never used
- **`smt_model_checker.h`** — Drop the now-unnecessary `#include
"ast/normal_forms/defined_names.h"`
- **`fingerprints.cpp`** — Collapse redundant tail in
`fingerprint_set::contains`:
  ```cpp
  // Before
  if (m_set.contains(d))
      return true;
  return false;

  // After
  return m_set.contains(d);
  ```

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

src/smt/fingerprints.cpp

@@ -104,9 +104,7 @@ namespace smt {
             return true;
         for (unsigned i = 0; i < num_args; ++i)
             d->m_args[i] = d->m_args[i]->get_root();
-        if (m_set.contains(d))
-            return true;
-        return false;
+        return m_set.contains(d);
     }
     
     void fingerprint_set::reset() {