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/smt_model_checker.cpp

@@ -258,7 +258,6 @@ namespace smt {
                 svector<symbol> names;
                 for (unsigned i = 0; i < f->get_arity(); ++i) 
                     names.push_back(symbol(i));
-                defined_names dn(m);
                 body = replace_value_from_ctx(body);
                 body = m.mk_lambda(sorts.size(), sorts.data(), names.data(), body);
                 sk_term = body;