caif: remove CAIF NETWORK LAYER

caif: remove CAIF NETWORK LAYER

Remove CAIF (Communication CPU to Application CPU Interface), the
ST-Ericsson modem protocol. The subsystem has been orphaned since 2013.
The last meaningful changes from the maintainers were in March 2013:
  a8c7687bf216 ("caif_virtio: Check that vringh_config is not null")
  b2273be8d2df ("caif_virtio: Use vringh_notify_enable correctly")
  0d2e1a2926b1 ("caif_virtio: Introduce caif over virtio")

Not-so-coincidentally, according to "the Internet" ST-Ericsson officially
shut down its modem joint venture in Aug 2013.

If anyone is using this code please yell!

In the 13 years since, the code has accumulated 200 non-merge commits,
of which 71 were cross-tree API changes, 21 carried Fixes: tags, and
the remaining ~110 were cleanups, doc conversions, treewide refactors,
and one partial removal (caif_hsi, ca75bcf0a83b).

We are still getting fixes to this code, in the last 10 days there were
3 reports on security@ about CAIF that I have been CCed on.

UAPI constants (AF_CAIF, ARPHRD_CAIF, N_CAIF, VIRTIO_ID_CAIF) and the
SELinux classmap entry are intentionally kept for ABI stability.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260416182829.1440262-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>