playbin3: Always register 'playbin3' element.
If the USE_PLAYBIN3=1 env var is set, we want to replace playbin with playbin3, but separate to that, we always want to register the 'playbin3' element so that applications which explicitly use playbin3 work regardless of the env var.
This fixes USE_PLAYBIN3=1 gst-validate-launcher
, for example.