[simple theme] remove '-webkit-border-radius' property

[1] https://github.com/searxng/searxng/pull/537#issuecomment-976582675

Suggested-by: @dalf [1]
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

searx/static/themes/simple/src/less/mixins.less

@@ -11,17 +11,14 @@
 }
 
 .rounded-corners (@radius: 10px) {
-  -webkit-border-radius: @radius;
   border-radius: @radius;
 }
 
 .rounded-right-corners (@radius: 0 10px 10px 0) {
-  -webkit-border-radius: @radius;
   border-radius: @radius;
 }
 
 .rounded-corners-tiny (@radius: 5px) {
-  -webkit-border-radius: @radius;
   border-radius: @radius;
 }