Remove get_number_of_channels

[h-mayorquin]

Follow up on:
#386

Now that we no longer have a channel as an argument of get_frames and we are sticking to the one extractor per channel argument I don't think it makes sense to have the get_number_of_channels method:

roiextractors/src/roiextractors/imagingextractor.py

Lines 74 to 83 in 10e56ee

	@abstractmethod
	def get_num_channels(self) -> int:
	"""Get the total number of active channels in the recording.
	Returns
	-------
	num_channels: int
	Integer count of number of channels.
	"""
	pass

I think we should remove it