Fix release build for MyRocks by using debug-only code only in debug builds

Summary: MyRocks release integration build breaks because we treat warnings caused by unused variables as errors. Variable `edit` is only used in debug builds. Therefore we need to guard it using `#ifndef NDEBUG` check.

Test Plan:
- `[p]arc diff --preview` for the default validation.
- Verify that release build fails before this fix and passes after applying it.

Reviewers: andrewkr, sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D60423

db/version_set.cc

@@ -2160,10 +2160,12 @@ Status VersionSet::LogAndApply(ColumnFamilyData* column_family_data,
   if (num_edits == 0) {
     return Status::OK();
   } else if (num_edits > 1) {
+#ifndef NDEBUG
     // no group commits for column family add or drop
     for (auto& edit : edit_list) {
       assert(!edit->IsColumnFamilyManipulation());
     }
+#endif
   }
 
   // column_family_data can be nullptr only if this is column_family_add.