netlink: specs: rt-link: remove if-netnsid from attr list

if-netnsid an alias to target-netnsid:

  IFLA_TARGET_NETNSID = IFLA_IF_NETNSID, /* new alias */

We don't have a definition for this attr.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250418021706.1967583-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/rt-link.yaml

@@ -2460,7 +2460,6 @@ operations:
             - xdp
             - event
             - new-netnsid
-            - if-netnsid
             - target-netnsid
             - carrier-up-count
             - carrier-down-count