Remove erroneous validation

@last_4_digits was replaced with a getter/setter that works with
@last_digits, and which already has validation.

Author: ugexe

lib/minfraud/components/credit_card.rb

@@ -116,7 +116,6 @@ def validate
         validate_telephone_country_code('bank_phone_country_code', @bank_phone_country_code)
         validate_regex('issuer_id_number', /\A(?:[0-9]{6}|[0-9]{8})\z/, @issuer_id_number)
         validate_regex('last_digits', /\A(?:[0-9]{2}|[0-9]{4})\z/, @last_digits)
-        validate_regex('last_4_digits', /\A(?:[0-9]{2}|[0-9]{4})\z/, @last_4_digits)
         validate_string('bank_name', 255, @bank_name)
         validate_string('bank_phone_number', 255, @bank_phone_number)
         validate_string('avs_result', 1, @avs_result)