staging: iio: Remove unused bit_count from struct iio_scan_el

The job this was intended to do (never implemented) is now done
by explicit definition of _type attributes in all drivers

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Manuel Stahl <manuel.stahl@iis.fraunhofer.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>