we don't need no discs for li's in wiki tocs

Table of contents in wiki files (included via the `{{toc}}` macro) had an unusual styling.

before | after
---------|------
![selection_139](https://cloud.githubusercontent.com/assets/206108/2929269/82bdd7bc-d787-11e3-8d35-c18297c61a72.png)|![selection_140](https://cloud.githubusercontent.com/assets/206108/2929272/864519fe-d787-11e3-96d5-c0a6aaa91d8f.png)


[``* `#7384` Headlines in wiki table of content are broken``](https://www.openproject.org/work_packages/7384)