s390/net: Remove NETIUCV device driver

s390/net: Remove NETIUCV device driver

The netiucv driver creates TCP/IP interfaces over IUCV between Linux
guests on z/VM and other z/VM entities.

Rationale for removal:
- NETIUCV connections are only supported for compatibility with
  earlier versions and not to be used for new network setups,
  since at least Linux kernel 4.0.
- No known active users, use cases, or product dependencies
- The driver is no longer relevant for z/VM networking;
  preferred methods include:
	* Device pass-through (e.g., OSA, RoCE)
	* z/VM Virtual Switch (VSWITCH)

The IUCV mechanism itself remains supported and is actively used
via AF_IUCV, hvc_iucv, and smsg_iucv.

Signed-off-by: Nagamani PV <nagamani@linux.ibm.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250715074210.3999296-1-wintera@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>