xfrm: update doc about xfrm[46]_gc_thresh

Those entries are not used anymore.

CC: Florian Westphal <fw@strlen.de>
Fixes: 09c75704 ("xfrm: remove flow cache")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>