Remove str-truncated-60 class from common.scss

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

app/assets/stylesheets/framework/common.scss

@@ -121,10 +121,6 @@ hr {
     @include str-truncated(30%);
   }
 
-  &-60 {
-    @include str-truncated(60%);
-  }
-
   &-100 {
     @include str-truncated(100%);
   }