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
@phproberto Should there even be a thead if there is no header to be shown?