fs/netfs: remove unused source NETFS_INVALID_WRITE

fs/netfs: remove unused source NETFS_INVALID_WRITE

This enum choice was added by commit 16af134ca4b7 ("netfs: Extend the
netfs_io_*request structs to handle writes") and its only user was
later removed by commit c245868524cc ("netfs: Remove the old writeback
code").

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/20250519134813.2975312-4-dhowells@redhat.com
cc: Paulo Alcantara <pc@manguebit.com>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>