[LibWebRTC] Remove non-existent file from sources after 264715@main

https://bugs.webkit.org/show_bug.cgi?id=257307

Reviewed by Eric Carlson.

Compilation was failing because file:

  - abseil-cpp/absl/synchronization/internal/waiter.cc

was removed after abseil update in 264715@main.

* Source/ThirdParty/libwebrtc/CMakeLists.txt: Remove file 'waiter.cc'
  from LibWebRTC sources.

Canonical link: https://commits.webkit.org/264822@main

Author: dpino

Source/ThirdParty/libwebrtc/CMakeLists.txt

@@ -138,7 +138,6 @@ set(webrtc_SOURCES
     Source/third_party/abseil-cpp/absl/synchronization/internal/create_thread_identity.cc
     Source/third_party/abseil-cpp/absl/synchronization/internal/graphcycles.cc
     Source/third_party/abseil-cpp/absl/synchronization/internal/per_thread_sem.cc
-    Source/third_party/abseil-cpp/absl/synchronization/internal/waiter.cc
     Source/third_party/abseil-cpp/absl/synchronization/mutex.cc
     Source/third_party/abseil-cpp/absl/synchronization/notification.cc
     Source/third_party/abseil-cpp/absl/time/civil_time.cc