maint: Remove unused structure in update path (#3833)

Signed-off-by: Julien Jerphanion <git@jjerphan.xyz>

libmamba/src/api/update.cpp

@@ -171,12 +171,6 @@ namespace mamba
         }
         PrefixData& prefix_data = exp_prefix_data.value();
 
-        std::vector<std::string> prefix_pkgs;
-        for (auto& it : prefix_data.records())
-        {
-            prefix_pkgs.push_back(it.first);
-        }
-
         load_installed_packages_in_database(ctx, db, prefix_data);
 
         auto request = create_update_request(prefix_data, raw_update_specs, update_params);