Update 0003-polymorphic-slot-definitions.md (#2044)

Removed `include ViewComponent::PolymorphicSlots` as this is not necessary anymore

Author: gonzric1

docs/adrs/0003-polymorphic-slot-definitions.md

@@ -52,7 +52,6 @@ Here's how the `Item` sub-component of the list example above would be implement
 
 ```ruby
 class Item < ViewComponent::Base
-  include ViewComponent::PolymorphicSlots
 
   renders_one :leading_visual, types: {
     icon: IconComponent, avatar: AvatarComponent