mx sstable reader: remove row_body_missing_columns_label
mx sstable reader: remove row_body_missing_columns_label

row_body_missing_columns_label is only reached from one
goto, or consecutively, so the code omitted by goto can be
ommited by an if instead (or else).

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla