staging: remove ashmem

The mainline replacement for ashmem is memfd, so remove the legacy
code from drivers/staging/

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220315123457.2354812-1-hch@lst.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>