Revert "ipv4: Clarify in docs that accept_local requires rp_filter."

This reverts commit c801e3cc ("ipv4: Clarify in docs that accept_local requires rp_filter.").
It is not needed anymore since commit 1dced6a8

 ("ipv4: Restore accept_local behaviour in fib_validate_source()").

Suggested-by: Julian Anastasov <ja@ssi.bg>
Cc: Gregory Detal <gregory.detal@uclouvain.be>
Cc: Christoph Paasch <christoph.paasch@uclouvain.be>
Cc: Hannes Frederic Sowa <hannes@redhat.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Sébastien Barré <sebastien.barre@uclouvain.be>
Signed-off-by: David S. Miller <davem@davemloft.net>