NO-JIRA: Removed incorrect rdoc for Messenger accept and reject.

The rdocs still listed flag as an argument even though no such argument
is accepted in the methods.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1525958 13f79535-47bb-0310-9956-ffa450edef68
PROTON-427: Removed the flag argument from Messenger.settle

Made the API more consistent with the accept and reject methods.

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1525959 13f79535-47bb-0310-9956-ffa450edef68
NO-JIRA: Removed the unused valid_flag? method from Qpid::Proton::Tracker

git-svn-id: https://svn.apache.org/repos/asf/qpid/proton/trunk@1525960 13f79535-47bb-0310-9956-ffa450edef68