iio: buffer-dmaengine: remove non managed alloc/free

This is to encourage the use of devm_iio_dmaengine_buffer_alloc().
Currently the managed version of the DMAEngine buffer alloc is the only
function used from this part of the framework.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20200923121810.944075-1-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>