Remove unused vacuum-truncate-auto injection point

Hi,

While looking at these injection points I noticed vacuum-truncate-auto
in vacuum_rel() can never fire:

```
#ifdef USE_INJECTION_POINTS
if (params.truncate == VACOPTVALUE_AUTO)
INJECTION_POINT("vacuum-truncate-auto", NULL);
else if (params.truncate == VACOPTVALUE_DISABLED)
INJECTION_POINT("vacuum-truncate-disabled", NULL);
else if (params.truncate == VACOPTVALUE_ENABLED)
INJECTION_POINT("vacuum-truncate-enabled", NULL);
#endif
```

truncate is a boolean and can never be VACOPTVALUE_AUTO

```
Assert(params->truncate != VACOPTVALUE_UNSPECIFIED &&
params->truncate != VACOPTVALUE_AUTO);
```

This looks like an oversight from 661643dedad, perhaps to mirror
index_cleanup?

```
if (params.index_cleanup == VACOPTVALUE_AUTO)
INJECTION_POINT("vacuum-index-cleanup-auto", NULL);
```

Small cleanup attached.

--
Sami Imseih
Amazon Web Services (AWS)