[Train Only] Drop openstacksdk-functional job

openstacksdk-functional job is broken since
heat stable/train branches are dropped after [1].

openstacksdk train gate is also not maintained[2],
let's drop the job from neutron project layout.

[1] https://review.opendev.org/c/openstack/releases/+/871857
[2] https://review.opendev.org/c/openstack/openstacksdk/+/747689

Closes-Bug: #2020363
Change-Id: I8b9d56430e6ceeb102bd323a3c3142f07306fd76

.zuul.yaml

@@ -36,7 +36,6 @@
               - ^tools/.*$
               - ^tox.ini$
         - tempest-integrated-networking
-        - openstacksdk-functional-devstack
         - grenade:
             irrelevant-files: *tempest-irrelevant-files
         - grenade-py3:
@@ -55,7 +54,6 @@
         - neutron-grenade
         - neutron-tempest-slow-py3
         - tempest-integrated-networking
-        - openstacksdk-functional-devstack
         - grenade:
             irrelevant-files: *tempest-irrelevant-files
         - grenade-py3: