tests: remove rtpaux test

The initial mission statement for this test was:

* demonstrate usage of the request-aux-* signals in rtpbin
* test the rtx elements

We have examples that serve the first use case, and better
(harnessed) tests for the second use case.

This test is slow and racy, it served its purpose but can now
be removed.

Fixes #533
6 jobs for rtpaux_test_remove in 31 minutes and 20 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #66541
gst indent

00:00:32

passed #66540
manifest

00:00:31

 
  Build
passed #66542
build fedora x86_64

00:04:03

 
  Test
passed #66543
check fedora

00:16:26

 
  Full Builds
passed #66544
build android arm64 api28

00:02:21

passed #66545
build cerbero fedora x86_64

00:09:47