Remove invalid css syntax.

This line can safely be removed since it does not impact
styling (nothing is being supplied to calculate).

Reference [here](http://www.w3schools.com/cssref/func_calc.asp)

lms/static/sass/shared/_footer-edx.scss

@@ -178,7 +178,6 @@ footer#footer-edx-v3 {
     }
 
     .social-media-links {
-      width: calc();
       margin-bottom: 40px;
     }
   }