droiddec: drop our custom negotiate and decide_allocation functions
droiddec: drop our custom negotiate and decide_allocation functions

The default provided by GstVideoDecoder is more that enough after we changed the format to YV12

mlehtima/gst-droid,mlehtima/gst-droid,foolab/gst-droid,foolab/gst-droid,sailfishos/gst-droid