Remove BigDecimal#to_d

This was backported for Ruby 1.8 support and is no longer needed.

Signed-off-by: David Celis <me@davidcel.is>

activesupport/lib/active_support/core_ext/big_decimal/conversions.rb

@@ -2,13 +2,6 @@ require 'bigdecimal'
 require 'bigdecimal/util'
 
 class BigDecimal
-  # Backport this method if it doesn't exist
-  unless method_defined?(:to_d)
-    def to_d
-      self
-    end
-  end
-
   DEFAULT_STRING_FORMAT = 'F'
   def to_formatted_s(*args)
     if args[0].is_a?(Symbol)

activesupport/test/core_ext/bigdecimal_test.rb

@@ -2,11 +2,6 @@ require 'abstract_unit'
 require 'active_support/core_ext/big_decimal'
 
 class BigDecimalTest < ActiveSupport::TestCase
-  def test_to_d
-    bd = BigDecimal.new '10'
-    assert_equal bd, bd.to_d
-  end
-
   def test_to_s
     bd = BigDecimal.new '0.01'
     assert_equal '0.01', bd.to_s