The way an application is represented in Unity is defined in its
.desktop file. In case we do not want an icon to appear in the Unity Launcher we may call an application from a .desktop file where we do not reference an icon, e.g.
Name=Calculator No Icon
Comment=Perform arithmetic, scientific or financial calculations
You can test this by saving this example in
~/.local/share/applications/<anyname>.desktop. Give this file executable permission to be able to run it from double click in Nautilus. Notice that the icon in the Launcher will be replaced by an empty space.
There is no guarantee that removing the
Icon= reference in its .desktop file prevents showing an icon as every window may define an icon representation at runtime. This can be seen when we remove the
/usr/share/applications/gcalctool.desktop file. Still gcalctool will show its icon in the Launcher when run from a terminal.
In any case we are not able to remove the empty placeholder for a graphical application that opens a window on the desktop.