fcntl: remove unused VALID_UPGRADE_FLAGS

fcntl: remove unused VALID_UPGRADE_FLAGS

We currently do not maky use of this feature and should we implement
something like this in the future it's trivial to add it back.

Link: https://lore.kernel.org/r/20210528092417.3942079-2-brauner@kernel.org
Cc: Christoph Hellwig <hch@lst.de>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Suggested-by: Richard Guy Briggs <rgb@redhat.com>
Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>