Revert "Don't free memory in generator"

This reverts commit 2ae033f.

benchmarks/libseff/generator/main.c

@@ -97,9 +97,8 @@ static int64_t run(int64_t n) {
   // Sum generated values
   int64_t result = sum(generator);
 
-  // NOTE: if larger program, should free tree memory:
-  // i.e. `freeTree(tree);`
-  // But since the program just exits, memory will be freed anyway.
+  // Free memory
+  freeTree(tree);
   return result;
 }