RE: Remove obsolate comments from 047_checkpoint_physical_slot

Dear hackers,

> While reviewing others, I found $SUBJECT.

I found another cleanup point related with this. In CreateCheckPoint():

```
#ifdef USE_INJECTION_POINTS
    INJECTION_POINT("checkpoint-before-old-wal-removal");
#endif
```

Here USE_INJECTION_POINTS check is not needed. If the feature is disabled,
the macro function would be ((void) name). IIUC, we are using the macro if if-branch
exists.

E.g., another injection point "create-restart-point" does not have the #ifdef part.

Best regards,
Hayato Kuroda
FUJITSU LIMITED