[sigslot] - Remove SignalDestroyed from PortAllocatorSession.

Bug: webrtc:11943
Change-Id: I2d615dc08627789b759f5c41b564ead212cf672e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/227966
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34672}

p2p/base/port_allocator.h

diff --git a/p2p/base/port_allocator.h b/p2p/base/port_allocator.h
index c1d583f..78b0c70 100644
--- a/p2p/base/port_allocator.h
+++ b/p2p/base/port_allocator.h
@@ -283,7 +283,6 @@
 
   virtual uint32_t generation();
   virtual void set_generation(uint32_t generation);
-  sigslot::signal1<PortAllocatorSession*> SignalDestroyed;
 
  protected:
   // This method is called when a pooled session (which doesn't have these