generate_packets.py: clean up string formatting in helpers for Variant.get_receive()
Part of #44577. Move the %-formatting of the outputs of Variant.get_delta_receive_body() and Field.get_get() in Variant.get_receive() into those methods and turn them into appropriate str.format() calls.
Part of #44577. Move the %-formatting of the outputs of Variant.get_delta_receive_body() and Field.get_get() in Variant.get_receive() into those methods and turn them into appropriate str.format() calls.