This interface and its implementations have been replaced by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}