xfrm: policy: remove set but not used variable 'priority'

Fixes gcc '-Wunused-but-set-variable' warning:

net/xfrm/xfrm_policy.c: In function 'xfrm_policy_lookup_bytype':
net/xfrm/xfrm_policy.c:2079:6: warning:
 variable 'priority' set but not used [-Wunused-but-set-variable]

It not used since commit 6be3b0db

 ("xfrm: policy: add inexact policy
search tree infrastructure")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>