net: sysctl: remove redundant #ifdef CONFIG_NET

CONFIG_NET is a bool option, and this file is compiled only when
CONFIG_NET=y.

Remove #ifdef CONFIG_NET, which we know it is always met.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20210125231421.105936-1-masahiroy@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>