I have already answered your support ticket on the subject. For convenience I will paste my answer here as well.
You can remove the icon for the custom button added to the toolbar of RadFileExplorer using the following CSS where customButton is CssClass applied to the custom button:
.RadFileExplorer .RadToolBar .customButton .rtbText
In addition, while in the solution provided by Shinu the CSS selector is slightly different, it is still valid one, however, it will not remove the extra space reserved for the image.
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