staging:iio: Remove declaration for non existing function

The declaration for iio_buffer_deinit has been around for quite some time, but
the function itself has never been added. So remove the declaration.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>