fs/netfs: remove unused flag NETFS_RREQ_DONT_UNLOCK_FOLIOS

NETFS_RREQ_DONT_UNLOCK_FOLIOS has never been used ever since it was
added by commit 3d3c9504 ("netfs: Provide readahead and readpage
netfs helpers").

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/20250519134813.2975312-11-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>