remove all useless header comments (#3363)

see #3358

used command: `find ./ -type f -exec perl -i -0pe
's/\/\*\s*magicmirror.*?\*\/\s*//si' {} \;`

This is a first draft, I think we should preserve some of the comments.

css/custom.css.sample

@@ -1,10 +1,8 @@
-/* MagicMirror² Custom CSS Sample
+/* Custom CSS Sample
  *
  * Change color and fonts here.
  *
  * Beware that properties cannot be unitless, so for example write '--gap-body: 0px;' instead of just '--gap-body: 0;'
- *
- * MIT Licensed.
  */
 
 /* Uncomment and adjust accordingly if you want to import another font from the google-fonts-api: */
@@ -18,7 +16,7 @@
 
   --font-primary: "Roboto Condensed";
   --font-secondary: "Roboto";
-  
+
   --font-size: 20px;
   --font-size-small: 0.75rem;
 
@@ -26,6 +24,6 @@
   --gap-body-right: 60px;
   --gap-body-bottom: 60px;
   --gap-body-left: 60px;
-  
+
   --gap-modules: 30px;
 }