Remove the file->last_completed_pos variable

Remove the file->last_completed_pos variable

This tracks where we last did IO to this file, however with file
sharing that may break. So just remove this optimization, it'll
cost an extra lseek() for the sync engine but doesn't do much else
outside of that.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>