udpencap: Remove Obsolete UDP_ENCAP_ESPINUDP_NON_IKE Support

The UDP_ENCAP_ESPINUDP_NON_IKE mode, introduced into the Linux kernel
in 2004 [2], has remained inactive and obsolete for an extended period.

This mode was originally defined in an early version of an IETF draft
[1] from 2001. By the time it was integrated into the kernel in 2004 [2],
it had already been replaced by UDP_ENCAP_ESPINUDP [3] in later
versions of draft-ietf-ipsec-udp-encaps, particularly in version 06.

Over time, UDP_ENCAP_ESPINUDP_NON_IKE has lost its relevance, with no
known use cases.

With this commit, we remove support for UDP_ENCAP_ESPINUDP_NON_IKE,
simplifying the codebase and eliminating unnecessary complexity.
Kernel will return an error -ENOPROTOOPT if the userspace tries to set
this option.

References:
[1] https://datatracker.ietf.org/doc/html/draft-ietf-ipsec-udp-encaps-00.txt



[2] Commit that added UDP_ENCAP_ESPINUDP_NON_IKE to the Linux historic
    repository.

    Author: Andreas Gruenbacher <agruen@suse.de>
    Date: Fri Apr 9 01:47:47 2004 -0700

   [IPSEC]: Support draft-ietf-ipsec-udp-encaps-00/01, some ipec impls need it.

[3] Commit that added UDP_ENCAP_ESPINUDP to the Linux historic
    repository.

    Author: Derek Atkins <derek@ihtfp.com>
    Date: Wed Apr 2 13:21:02 2003 -0800

    [IPSEC]: Implement UDP Encapsulation framework.

Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>