fs/netfs: remove unused flag NETFS_SREQ_SEEK_DATA_READ

This flag was added by commit 3d3c9504 ("netfs: Provide readahead
and readpage netfs helpers") but its only user was removed by commit
86b374d0 ("netfs: Remove fs/netfs/io.c").

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/20250519134813.2975312-3-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>