read_dig_captrack ignores eeg ref

see this:
https://github.com/mne-tools/mne-python/blob/29b05f2f593f997168d2f29a41829f68f56ad5df/mne/channels/_dig_montage_utils.py#L185-L189

	# Need to prune "GND" and "REF": these are not included in the raw
	# data and will raise errors when we try to do raw.set_montage(...)
	# XXX eventually this should be stored in ch['loc'][3:6]
	# but we don't currently have such capabilities here
	if name in ['GND', 'REF']:

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