Hi
You are not able to do this. I saw your screen shot too. This is not possible. To remove it you need code customization. The code is inside interface->inc->structure->content-extension.php
From there which template are you choosing. i think its blog image large. There inside content-extension.php there is the content you can remove but we recommend not to change the code because while updating to to our new version all your customization code will be lost. So better make child theme and unhook the function first and you can edit the text.
http://codex.wordpress.org/Child_Themes
Thank you
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