config: mark column_index_size_in_kb as Used
config: mark column_index_size_in_kb as Used

Fixes #2681
Message-Id: <615e1ce61479f3b776587e970c50466809d3481d@scylladb.com>

duarten/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,scylladb/scylla