mutation_partition: drop cells from dropped_columns at upgrade
mutation_partition: drop cells from dropped_columns at upgrade

Signed-off-by: Paweł Dziepak <e29bb3265626c60caf76d587cabdfcd8b379ef80@scylladb.com>
[tgrabiec: Merged the changes into converting_mutation_partition_applied]

avikivity/scylla,scylladb/scylla,raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,kjniemi/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla