Review supposedly dead code in RxD.

[1uc]

The following lines are suspected to be dead code:

nrn/share/lib/python/neuron/rxd/rxd.py

Lines 622 to 636 in 7b9e1be

	def _matrix_to_rxd_sparse(m):
	"""precondition: assumes m a numpy array"""
	nonzero_i, nonzero_j = list(zip(*list(m.keys())))
	nonzero_values = numpy.ascontiguousarray(list(m.values()), dtype=float)
	# number of rows
	n = m.shape[1]
	return (
	n,
	len(nonzero_i),
	numpy.ascontiguousarray(nonzero_i, dtype=ctypes.c_long),
	numpy.ascontiguousarray(nonzero_j, dtype=ctypes.c_long),
	nonzero_values,
	)

This ticket extracts a task from review comments:
https://github.com/neuronsimulator/nrn/pull/3040/files/7b9e1be83f16e36ec5e60d3da0cda1731938ae17#r1729011132

CC: @ramcdougal, @ceciliaromaro