Forms CSS: fix .form-addon button alignment in Chrome

Fixes the alignment of the [.form-addon button](http://wet-boew.github.io/wet-boew/demos/grids/grid-form-en.html#buttons) in Chrome and removes an extra min-height rule.

![form-addon](https://f.cloud.github.com/assets/2110107/1994777/35e06344-84f6-11e3-9344-39336b51baa3.png)