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/BUILD.gn

@@ -918,6 +918,7 @@ if (is_ios || is_mac) {
         "../api/video:video_frame",
         "../api/video_codecs:video_codecs_api",
         "../common_video",
+        "../logging:rtc_event_log_impl_base",
         "../media:rtc_media_base",
         "../modules:module_api",
         "../modules/audio_device:audio_device_api",