Actually, I finally found out how from Softonic. It's very simple and it's hard to believe that this answer is not easily accessible on the Internet:
In order to remove (uninstall) the "Softonic Toolbar" from your "Internet Explorer" follow these steps.
First make sure you have the "Menu bar" in your "Internet Explorer" activated. If it is not active, right-click somewhere on the grey area on the "Internet Explorer" window and make sure the "Menu Bar" option is checked.
When the "Menu Bar" is visible, click on "Tools" at the far right, then click on "Manage Add-ons". Click on 'Enable or Disable Add-Ons'. In the list of "Add-ons" select "Softonic English Toolbar". Look below and click on the 'Disable' button. You may need to restart your computer for it to take effect.