Remove rtc_base:rtc_base_approved_generic.

After landing https://webrtc-review.googlesource.com/c/src/+/104802, it
is finally possible to remove the complexity behind
rtc_base:rtc_base_approved and switch back to one build target.

The long term vision is to remove it too, in favor of smaller and more
focues build targets.

Bug: webrtc:9838
Change-Id: Ib98dfae103a20edb8c8b6706d376ad4f3c992886
Reviewed-on: https://webrtc-review.googlesource.com/c/105041
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25093}