Revert "bridge: Add extack warning when enabling STP in netns."

Revert "bridge: Add extack warning when enabling STP in netns."

This reverts commit 56a16035bb6effb37177867cea94c13a8382f745.

Since the previous commit, STP works on bridge in netns.

  # unshare -n
  # ip link add br0 type bridge
  # ip link add veth0 type veth peer name veth1

  # ip link set veth0 master br0 up
  [   50.558135] br0: port 1(veth0) entered blocking state
  [   50.558366] br0: port 1(veth0) entered disabled state
  [   50.558798] veth0: entered allmulticast mode
  [   50.564401] veth0: entered promiscuous mode

  # ip link set veth1 master br0 up
  [   54.215487] br0: port 2(veth1) entered blocking state
  [   54.215657] br0: port 2(veth1) entered disabled state
  [   54.215848] veth1: entered allmulticast mode
  [   54.219577] veth1: entered promiscuous mode

  # ip link set br0 type bridge stp_state 1
  # ip link set br0 up
  [   61.960726] br0: port 2(veth1) entered blocking state
  [   61.961097] br0: port 2(veth1) entered listening state
  [   61.961495] br0: port 1(veth0) entered blocking state
  [   61.961653] br0: port 1(veth0) entered listening state
  [   63.998835] br0: port 2(veth1) entered blocking state
  [   77.437113] br0: port 1(veth0) entered learning state
  [   86.653501] br0: received packet on veth0 with own address as source address (addr:6e:0f:e7:6f:5f:5f, vlan:0)
  [   92.797095] br0: port 1(veth0) entered forwarding state
  [   92.797398] br0: topology change detected, propagating

Let's remove the warning.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>