Remove has_slots from a class that does not need it

sigslot::has_slots<> is only needed in the class that listens to
signals, not the class that sends it.

Bug: webrtc:11943
Change-Id: I387057c7e1f999a260eade7b5e38a0df5ee0f40a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/192382
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32574}

pc/webrtc_session_description_factory.h

@@ -37,8 +37,7 @@ namespace webrtc {
 
 // DTLS certificate request callback class.
 class WebRtcCertificateGeneratorCallback
-    : public rtc::RTCCertificateGeneratorCallback,
-      public sigslot::has_slots<> {
+    : public rtc::RTCCertificateGeneratorCallback {
  public:
   // |rtc::RTCCertificateGeneratorCallback| overrides.
   void OnSuccess(