Alignment: categories block forces aligncenter regardless of what is selected in the toolbar

[designsimply]

Moved from #4010 h/t @bradyvercher.

Steps to reproduce:

1. Insert a categories block and publish (the class is aligncenter).
2. Align it right and save (the class is alignright).
3. Unselect alignment and save (the class is aligncenter).

Result:

The PHP render method forces an aligncenter class to be applied even when an alignment button isn't selected in the toolbar.

^{Seen at http://alittletestblog.com/2018/07/11/categories-block/ running WordPress 4.9.7 and Gutenberg 3.2.0 using Firefox 61.0.1 on macOS 10.13.5.}