Sometimes after uninstalling software the icon is still there in your unity dash.  

So a question pops up in my mind , How can it be completely removed from my system ? 

Dash picks up its entries from .desktop files placed , so if we remove it from there it will solve the problem. 
Steps to remove :
Delete .desktop file corresponding to your program (in this case skype.desktop)  from
 1.   /usr/share/applications and2.~/.local/share/applications.