net: remove ISDN subsystem and Bluetooth CMTP

net: remove ISDN subsystem and Bluetooth CMTP

Remove the ISDN (mISDN, CAPI) subsystem and Bluetooth CMTP protocol
from the kernel tree.

ISDN is a pretty old technology and it's unclear whether anyone still
uses it. I went over the last few years of git history and all the
commits are either tree-wide conversions or syzbot/static analyzer
fixes.

When we discussed removal in the past IIRC there were some concerns
about ISDN still being used in parts of Germany. Unfortunately, the
code base is quite old, none of the current maintainers are familiar
with it and AI tools will have a field day finding bugs here.

Delete this code and preserve it in an out-of-tree repository
for any remaining users:
https://github.com/linux-netdev/mod-orphan

UAPI constants AF_ISDN/PF_ISDN and the SELinux isdn_socket class
are preserved for ABI stability, but the rest of uAPI is removed.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260421022108.1299678-1-kuba@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>