[libcalamares] Remove redundant default: in case

- the switch handles all values of the enum and the compiler should
   be smart enough to know that (therefore default isn't needed,
   nor the return afterwards).

src/libcalamares/geoip/Handler.cpp

@@ -112,8 +112,6 @@ create_interface( Handler::Type t, const QString& selector )
 #else
             return nullptr;
 #endif
-        default:  // there are no others
-            return nullptr;
     }
 }