xfs_io/encrypt: remove unimplemented encryption modes

xfs_io/encrypt: remove unimplemented encryption modes

Although mode numbers were originally reserved for AES-256-GCM and
AES-256-CBC, these were never implemented in the kernel, and there are
no plans to do so anymore.  These mode numbers may be used for something
else in the future.  Also, xfstests (the only known user of the xfs_io
encryption commands) doesn't try to use them.  Finally, most of the
fscrypt constants have been given new names in the UAPI header, but the
unused constants have not been given new names since userspace should
just stop referencing them instead.

So remove them from xfs_io.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>