Remove unused mutation_and_reader::less_compare and operator<
Remove unused mutation_and_reader::less_compare and operator<

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