Remove example file

This documentation should be in the guides.

Closes #16691

activemodel/examples/validations.rb

-require File.expand_path('../../../load_paths', __FILE__)
-require 'active_model'
-
-class Person
-  include ActiveModel::Conversion
-  include ActiveModel::Validations
-
-  validates :name, presence: true
-
-  attr_accessor :name
-
-  def initialize(attributes = {})
-    @name = attributes[:name]
-  end
-
-  def persist
-    @persisted = true
-  end
-
-  def persisted?
-    @persisted
-  end
-end
-
-person1 = Person.new
-p person1.valid? # => false
-p person1.errors.messages # => {:name=>["can't be blank"]}
-
-person2 = Person.new(name: 'matz')
-p person2.valid? # => true