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.

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.