Make the remote_bitrate_estimator build target private

Bug: webrtc:9808
Change-Id: I6f0c23841bbdc2999505b89888cf261c31536408
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/194330
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Björn Terelius <terelius@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32703}

modules/remote_bitrate_estimator/BUILD.gn

@@ -9,7 +9,6 @@
 import("../../webrtc.gni")
 
 rtc_library("remote_bitrate_estimator") {
-  visibility = [ "*" ]
   sources = [
     "aimd_rate_control.cc",
     "aimd_rate_control.h",

native-api.md

@@ -21,7 +21,6 @@ Legacy API directory                       | Including subdirectories?
 `modules/audio_processing/include`         | No
 `modules/congestion_controller/include`    | No
 `modules/include`                          | No
-`modules/remote_bitrate_estimator/include` | No
 `modules/rtp_rtcp/include`                 | No
 `modules/rtp_rtcp/source`                  | No
 `modules/utility/include`                  | No