Thank you, but it does not work!
Version theme_essential 2.8.1.8 (Build: 2015061902)
Moodle 2.8.7 (Build: 20150706)
FireBug:
<div id="yui_3_17_2_2_1441347531108_286" class="thecredit row-fluid">
I updated Essential theme to 2.8.1.8 today and I too got the footer message reappear.
I'd previously hidden it with: #page-footer .thecredit { visibility: hidden; }
So it appears something has changed in this latest version.
"If" you still want to remove it, along with your custom CSS, comment out line 234 in Essential's lib.php i.e.
// $customcss = str_replace('thecredit', 'themepagefooter', $customcss);