Remove support for reading YAML files

[aszlig]

We already introduced deprecation warnings back in 49b889b03eb5e39f4fc348966a989567d58, telling people that we're going to remove YAML in version 3.0.

With this, the removal is essentially done and we now no longer depend on yaml-cpp, nor do we need pkg-config or anything like that, because we essentially have zero (external) library dependencies except libc and libdl.

This of course also greatly simplifies our parsing code and also gets rid of a few tests that now became obsolete.