Remove redundant #add_child method

src/spectator/dsl/nested_example_group_builder.cr

@@ -3,10 +3,6 @@ module Spectator::DSL
     def initialize(@what : String)
     end
 
-    def add_child(child : Child) : Nil
-      @children << child
-    end
-
     def build(parent : ExampleGroup, sample_values : Internals::SampleValues) : NestedExampleGroup
       NestedExampleGroup.new(@what, parent, hooks).tap do |group|
         group.children = @children.map do |child|