neutron: fix build in stable branches

When [1] was committed, neutron stopped building for Train.
Analogous patch is proposed to other stable branches.

This patch removes neutron from upper-constraints.

Kolla master is affected directly because we build Train
for CentOS 7 atm.

1. https://review.opendev.org/#/c/697370/

Change-Id: I944e8e42fef1d359d767cbc6e1c13371ed753f31

docker/neutron/neutron-base/Dockerfile.j2

@@ -129,6 +129,7 @@ ADD plugins-archive /
 ] %}
 
 RUN ln -s neutron-base-source/* neutron \
+    && sed -i /^neutron=/d /requirements/upper-constraints.txt \
     && {{ macros.install_pip(neutron_base_pip_packages | customizable("pip_packages")) }} \
     && mkdir -p /etc/neutron /usr/share/neutron \
     && cp -r /neutron/etc/* /etc/neutron/ \