mptcp: drop unused rcu member in mptcp_pm_addr_entry

kfree_rcu() had been removed from pm_netlink.c, so this rcu field in
struct mptcp_pm_addr_entry became useless. Let's drop it.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Link: https://lore.kernel.org/r/20210802231914.54709-1-mathew.j.martineau@linux.intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>