netfs: Delete some xarray-wangling functions that aren't used

Delete some xarray-based buffer wangling functions that are intended for
use with bounce buffering, but aren't used because bounce-buffering got
deferred to a later patch series.  Now, however, the intention is to use
something other than an xarray to do this.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/20240620173137.610345-9-dhowells@redhat.com


Signed-off-by: Christian Brauner <brauner@kernel.org>