What is the permission necessary to drop a service in service-broker?
After I post this question, I just found my answer with some tricky way. I went to the left object explorer and delete the contract with it. but instead of submitting the form I click on the script button which produces the scripts. It was my mistake to drop a contract with drop service command. so the correct query to drop a contract is:

IF EXISTS(SELECT * FROM sys.service_contracts  WHERE NAME = '//Audit/Contract')
    DROP CONTRACT [//Audit/Contract]


I post this answer because maybe somebody encounter with this error and my guide can be useful to him/her.