sstables: mutation_reader: Don't try to read index to skip to static row
sstables: mutation_reader: Don't try to read index to skip to static row

Static row is always at the beginning, there's no point in doing
index lookups.

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