Remove default theme class

We can't scope the default theme like we can other themes because they use ::part and have a higher specificity, so this isn't very useful.

src/styles/shoelace.scss

@@ -1,8 +1,7 @@
 /*! Shoelace */
 @import 'mixins/make-color-palette';
 
-:root,
-.sl-theme-default {
+:root {
   //////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
   // Color tokens
   //////////////////////////////////////////////////////////////////////////////////////////////////////////////////////