tidy up redundant version related config checking based on proton-c version found

There is some redundant version specific config in the cmake files based on the proton-c version in use, at https://github.com/apache/qpid-cpp/blob/1.38.0/src/amqp.cmake#L29. The answer will always be fixed since https://github.com/apache/qpid-cpp/blob/1.38.0/src/amqp.cmake#L22 already requires a newer proton release than any of the versions checked for.