Remove swipe_ratio parameter when only one layer is visible

Fix #1409 

Ths PR remove the swipe_ratio parameter form the permalink when there is no layer to compare. 

So permalink with a swipe_ratio paremeter like in   #1409  shouldn't be possible, I guess if the user add it manually in the permalink he knows what he's doing.

[Test](http://mf-geoadmin3.dev.bgdi.ch/teo_swipe_remove)