refactor: remove dead code in CountWitnessSigOps

Found while reviewing #32840

The `nullptr` witness path was dead in normal code paths: removing it deletes unreachable logic.

Code coverage proof:
https://maflcko.github.io/b-c-cov/total.coverage/src/script/interpreter.cpp.gcov.html#L2135

src/test/fuzz/script.cpp

@@ -125,7 +125,7 @@ FUZZ_TARGET(script, .init = initialize_script)
             for (const auto& s : random_string_vector) {
                 wit.stack.emplace_back(s.begin(), s.end());
             }
-            (void)CountWitnessSigOps(script, *other_script, &wit, flags);
+            (void)CountWitnessSigOps(script, *other_script, wit, flags);
             wit.SetNull();
         }
     }