Clean up visibility and dependencies of RTC event log build targets.

- Remove visibility of encoder target.
- Remove unnecessary dependency on task_queue.
- Remove CreateRtcEventLogFactory() declaration from the rtc_event_log_api target
  since the function is not defined in that target.

Bug: None
Change-Id: Id9edee86f358d08ea063d62bd96e9653c5b06d55
Reviewed-on: https://webrtc-review.googlesource.com/c/116060
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Björn Terelius <terelius@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26215}

sdk/objc/DEPS

@@ -7,6 +7,7 @@ include_rules = [
   "+common_video/h264",
   "+common_video/include",
   "+common_video/libyuv/include",
+  "+logging/rtc_event_log/rtc_event_log_factory.h",
   "+media",
   "+modules/video_coding",
   "+pc",

sdk/objc/api/peerconnection/RTCPeerConnectionFactory.mm

@@ -30,6 +30,7 @@
 // is not smart enough to take the #ifdef into account.
 #include "api/audio_codecs/builtin_audio_decoder_factory.h"     // nogncheck
 #include "api/audio_codecs/builtin_audio_encoder_factory.h"     // nogncheck
+#include "logging/rtc_event_log/rtc_event_log_factory.h"
 #include "media/engine/convert_legacy_video_factory.h"          // nogncheck
 #include "modules/audio_device/include/audio_device.h"          // nogncheck
 #include "modules/audio_processing/include/audio_processing.h"  // nogncheck