use `proc->pgxactoff` as the value of `index` in `ProcArrayRemove()`

Hi,

Since we have introduced `pgxactoff` in [941697c3c1ae5d6ee153065adb96e1e63ee11224](https://github.com/postgres/postgres/commit/941697c3c1ae5d6ee153065adb96e1e63ee11224), and `pgxactoff` is always the index of `proc->pgprocno` in `procArray->pgprocnos`. So it seems that we could directly use `proc->pgxactoff` as the value of `index` in `ProcArrayRemove()`? My thought is to replace

```c
for (index = 0; index < arrayP->numProcs; index++)
{
if (arrayP->pgprocnos[index] == proc->pgprocno)
{
/* ... */
}
}
```

with

```c
index = proc->pgxactoff;
/* ... */
```

I would appreciate your help.