If you are in Joomla 3.4.1, the easiest way in my opinion is to override the css rule for the
.icon-eye.thememanager:before selector, using a custom.css inside the ISIS template css folder.
Just create the custom.css file, place it in:
administrator/templates/isis/css
and add your css override rules for the :before pseudo class.
It can be a display:none, or a override over the "content" attribute, e.g.:
.icon-eye.thememanager:before {content:"";}
You are done.
*Keep in mind, that this seems to be a new feature introduced in Joomla 3.4.1. It's unknown yet how this will evolve and what might be the effects of a Joomla update. Just to stay in the safe side, you may want to backup the custom css file before updating.