ipv6: replace IS_BUILTIN(CONFIG_IPV6) with IS_ENABLED(CONFIG_IPV6)

ipv6: replace IS_BUILTIN(CONFIG_IPV6) with IS_ENABLED(CONFIG_IPV6)

As IPv6 is built-in only, it does not make sense to continue using
IS_BUILTIN(CONFIG_IPV6). Therefore, replace it with IS_ENABLED() when
necessary and drop it if it isn't valid anymore.

Notice that there is still one instance related to ICMPv6, as it
requires more changes it will be handle separately.

Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Tested-by: Ricardo B. Marlière <rbm@suse.com>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20260325120928.15848-4-fmancera@suse.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>