backend-pipewire: remove linear modifier for now
As of PipeWire version 0.3.69, the gstpipewiresrc element uses the existence of a modifier as a trigger to select dmabuf memory, failing caps negotiation as we don't send DMA buffers yet.
Remove the linear modifier for now, to be added back when we add dmabuf support to the PipeWire backend. This allows testing the PipeWire backend with current GStreamer + PipeWire.
/cc @m.tretter