netfilter: conntrack: remove nf_conntrack_helper documentation

This toggle has been already remove by b1185090 ("netfilter: remove
nf_conntrack_helper sysctl and modparam toggles").

Remove the documentation entry for this toggle too.
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>

Documentation/networking/nf_conntrack-sysctl.rst

@@ -70,15 +70,6 @@ nf_conntrack_generic_timeout - INTEGER (seconds)
 	Default for generic timeout.  This refers to layer 4 unknown/unsupported
 	protocols.
 
-nf_conntrack_helper - BOOLEAN
-	- 0 - disabled (default)
-	- not 0 - enabled
-
-	Enable automatic conntrack helper assignment.
-	If disabled it is required to set up iptables rules to assign
-	helpers to connections.  See the CT target description in the
-	iptables-extensions(8) man page for further information.
-
 nf_conntrack_icmp_timeout - INTEGER (seconds)
 	default 30