tests: plugin: remove feature refcount assert
tests: plugin: remove feature refcount assert

This check fails if one, or more, tracers are loaded while running the
test. The new "leaks" tracer will be able to check for leaks anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=765052

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer