Revert "Speed up integer casting from DB"

This reverts commit 52fddcc.

52fddcc was to short-circuit `ensure_in_range` in `cast_value`. But that
caused a regression for empty string deserialization.

Since 7c6f393, `ensure_in_range` is moved into `serialize`. As 52fddcc
said, the absolute gain is quite small. So I've reverted that commit to
fix the regression.

activemodel/lib/active_model/type/integer.rb

@@ -18,11 +18,6 @@ def type
         :integer
       end
 
-      def deserialize(value)
-        return if value.nil?
-        value.to_i
-      end
-
       def serialize(value)
         result = super
         if result

activemodel/test/cases/type/integer_test.rb

@@ -54,7 +54,7 @@ class IntegerTest < ActiveModel::TestCase
         type = Type::Integer.new
         assert_nil type.cast("")
         assert_nil type.serialize("")
-        assert_equal 0, type.deserialize("")
+        assert_nil type.deserialize("")
       end
 
       test "changed?" do