esp6: remove redundant variable err

Variable err is being assigned a value that is not read, the assignment
is redundant and so is the variable. Remove it.

Cleans up clang scan warning:
net/ipv6/esp6_offload.c:64:7: warning: Although the value stored to 'err'
is used in the enclosing expression, the value is never actually read
from 'err' [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>