remove invalid HTML in Category List when Table headings 'of...

remove invalid HTML in Category List when Table headings 'off'

## Original Report

#### Steps to reproduce the issue
Menu item - Category List
Table headings switched off



#### Expected result

html:
tr class="cat-list-row0"
td class="list-title"
...

#### Actual result

html:
tr class="cat-list-row0" 
td headers="categorylist_header_title" class="list-title"

but no th tag in HTML output, so:
'A reference has been made to a non-existent ID. '

#### System information (as much as possible)

Joomla 3.3.6


#### Additional comments

HTML validation failed
if use class hidden, the border of table will be like this
![screenshot from 2014-10-20 16 50 42](https://cloud.githubusercontent.com/assets/8347187/4699264/b53fb380-583e-11e4-985e-4e5f2d5f9a28.png)
instead of this
![screenshot from 2014-10-20 16 50 02](https://cloud.githubusercontent.com/assets/8347187/4699281/e32b93fe-583e-11e4-8454-f7dffded7639.png)