Remove ActiveModel::Errors#add_on_present method.

We don't need to define a new method in ActiveMode::Errors for each
validatior.

See
https://github.com/rails/rails/commit/d72a07f1d1478db9daed847eadb35bfd840674f6#commitcomment-2325333

activemodel/lib/active_model/errors.rb

@@ -328,19 +328,6 @@ def add_on_blank(attributes, options = {})
       end
     end
 
-    # Will add an error message to each of the attributes in +attributes+ that
-    # is present (using Object#present?).
-    #
-    #   person.errors.add_on_present(:name)
-    #   person.errors.messages
-    #   # => { :name => ["must be blank"] }
-    def add_on_present(attributes, options = {})
-      Array(attributes).flatten.each do |attribute|
-        value = @base.send(:read_attribute_for_validation, attribute)
-        add(attribute, :not_blank, options) if value.present?
-      end
-    end
-
     # Returns +true+ if an error on the attribute with the given message is
     # present, +false+ otherwise. +message+ is treated the same as for +add+.
     #

activemodel/lib/active_model/validations/absence.rb

@@ -2,8 +2,8 @@ module ActiveModel
   module Validations
     # == Active Model Absence Validator
     class AbsenceValidator < EachValidator #:nodoc:
-      def validate(record)
-        record.errors.add_on_present(attributes, options)
+      def validate_each(record, attr_name, value)
+        record.errors.add(attr_name, :not_blank, options) if value.present?
       end
     end
 

activemodel/lib/active_model/validator.rb

@@ -110,7 +110,7 @@ def initialize(options)
     # Return the kind for this validator.
     #
     #   PresenceValidator.new.kind   # => :presence
-    #   UniquenessValidator.new.kind # => :uniqueness 
+    #   UniquenessValidator.new.kind # => :uniqueness
     def kind
       self.class.kind
     end