This interface and its implementations have been replaced by rtc::RTCCertificateGeneratorInterface. Removes dtlsidentitystore.h, updates .gyp/gn and removes old #includes. BUG=webrtc:5707, webrtc:5708 Review-Url: https://codereview.webrtc.org/2034013003 Cr-Commit-Position: refs/heads/master@{#13432}