asciidoc.conf: 'docinfo' attrib. warning for purehtml target

The inclusion of the `docinfo` attribute in `asciidoc.conf` leads to the following warning for `make purehtml`:

```
mw@nuc:~/work/dpmb$ make purehtml
asciidoc  debian-paketmanagement.txt
asciidoc: WARNING: version.txt: line 2: {include:/opt/mw/work/dpmb/debian-paketmanagement-docinfo.html}: file does not exist
```
