Leftovers of removed config option: dynamicusers

```
Arch Linux
Pikaur v1.33.2.r7.gddd4e7c6 
```

Undocumented .conf option,  `dynamicusers`, in manpage and help output present in `pikaur.conf` file:

```
....
[build]
dynamicusers = never
ignorearch = no
....
```