netfs: Clean up now-unused code

Delete now-unused code from netfslib, given the changes to the writeback
dispatch code and the combination of ->prepare_xxx() and ->issue_xxx()
methods.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org