mx sstable reader: remove row_body_marker_label
mx sstable reader: remove row_body_marker_label

row_body_marker_label is only reached from one goto inside an else
case, or consecutively, so the code omitted by goto can be moved
inside the corresponding if case.

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