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)