fix(disk_manager): remove redundant open file

[skyzh]

Signed-off-by: Alex Chi [email protected]

close https://github.com/cmu-db/bustub/issues/150

log_io_.open(log_name_, std::ios::binary | std::ios::trunc | std::ios::app | std::ios::out);

This line will always fail and doesn't take effect at all. We can directly create (or truncate) a file if we can't open it.