Revert last commit and add "unused" attribute to suppress warning

db/db_test.cc

@@ -2596,7 +2596,6 @@ class ChangeFilter : public CompactionFilter {
                       const Slice& value, std::string* new_value,
                       bool* value_changed) const override {
     assert(argv_ == 100);
-    argv_ = argv_ + 0;
     assert(new_value != nullptr);
     *new_value = NEW_VALUE;
     *value_changed = true;
@@ -2608,7 +2607,7 @@ class ChangeFilter : public CompactionFilter {
   }
 
  private:
-  const int argv_;
+  const int __attribute__((unused)) argv_;
 };
 
 class KeepFilterFactory : public CompactionFilterFactory {