however, if you want to delete that link then go to your admin panel, board configuration, board settings and remove the text from the box.
as to why you can't find it in the overall_header file:
I forgot to mention that the template files are a combination of html, css and php variables.
the varible will pull the required info from the database and replace the varible with the data and display it on the actual web page that the viewer sees.
In this case that variable in the overall_header.html file is
so, you can edit just about anything in the html/css but you shouldn't edit any of those variables without really knowing what you are doing.
many of those varibles are based on admin panel settings that you can change from the admin panel without any need to edit the actual files.