Remove wrong comment from `PushableWriter::SyncScratch()`: it does not change the current position (that was true in `PullableReader`).
Remove wrong comment from `PushableWriter::SyncScratch()`: it does not
change the current position (that was true in `PullableReader`).

PiperOrigin-RevId: 275858178

google/riegeli,google/riegeli,google/riegeli,google/riegeli