MIPS: Alchemy: Remove reverted CONFIG_NETLINK_MMAP from db1xxx_defconfig

Netlink mmap support and the relevant CONFIG_NETLINK_MMAP option have been
reverted by commit d1b4c689 ("netlink:
remove mmapped netlink support").

Remove the occurrence of CONFIG_NETLINK_MMAP from db1xxx_defconfig.

Signed-off-by: Nikitas Angelinas <nikitas.angelinas@gmail.com>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Florian Westphal <fw@strlen.de>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: Jiri Kosina <trivial@kernel.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>