netfilter: nf_tables: Remove unused nft_reduce_is_readonly()

Since commit 9e539c5b ("netfilter: nf_tables: disable expression
reduction infra") this is unused.

Signed-off-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>