Remove dtlsidentitystore.h from libjingle GYP/GN files.

This file contains an interface that is about to be removed
(https://codereview.webrtc.org/2034013003) since it is no longer used
by chromium (https://codereview.chromium.org/2033353002).

The build files have to be updated before the file is removed from the
webrtc repo or else the webrtc roll will cause chromium to break.

BUG=webrtc:5708, webrtc:5707
NOTRY=true

Review-Url: https://codereview.chromium.org/2120493002
Cr-Commit-Position: refs/heads/master@{#404614}

third_party/libjingle/BUILD.gn

@@ -250,7 +250,6 @@ if (enable_webrtc) {
       "../webrtc/api/audiotrack.h",
       "../webrtc/api/datachannel.cc",
       "../webrtc/api/datachannel.h",
-      "../webrtc/api/dtlsidentitystore.h",
       "../webrtc/api/dtmfsender.cc",
       "../webrtc/api/dtmfsender.h",
       "../webrtc/api/jsep.h",

third_party/libjingle/libjingle.gyp

@@ -218,7 +218,6 @@
             '<(DEPTH)/third_party/webrtc/api/audiotrack.h',
             '<(DEPTH)/third_party/webrtc/api/datachannel.cc',
             '<(DEPTH)/third_party/webrtc/api/datachannel.h',
-            '<(DEPTH)/third_party/webrtc/api/dtlsidentitystore.h',
             '<(DEPTH)/third_party/webrtc/api/dtmfsender.cc',
             '<(DEPTH)/third_party/webrtc/api/dtmfsender.h',
             '<(DEPTH)/third_party/webrtc/api/jsep.h',