Remove already done use-builtin-mock from goal

use-builtin-mock work is already done under:
https://review.opendev.org/q/topic:%22unittest.mock%22+(status:open%20OR%20status:merged)

We do not need to keep it in proposal goal doc.

Change-Id: Icc1e837c1de964caba505c1d7cfe0467ef88808e

goals/proposed/use-builtin-mock.rst

@@ -1,100 +0,0 @@
-=========================================
-Migrate from mock to bultin unittest.mock
-=========================================
-
-The external mock library was necessary in python versions less than 3.3.  The
-built in unittest.mock provides the functionality of the old mock library and
-also provides a more stable interface vs the constantly updating external mock
-library.
-
-
-Champion
-========
-
-* Matthew Thode <mthode@mthode.org> (prometheanfire)
-* Sean McGinnis <sean.mcginnis@gmail.com> (smcginnis)
-
-
-Gerrit Topic
-============
-
-To facilitate tracking, commits related to this goal should use the
-gerrit topic::
-
-  unittest.mock
-
-A new Story in https://storyboard.openstack.org/ will be created to track any
-related patch.
-
-
-Completion Criteria
-===================
-
-#. All projects with managed constraints do not use the mock library but use
-   the built in unittest.mock.
-
-
-References
-==========
-
-The main refrence is upstream documentation located at [1]_
-An example review can be found in [2]_
-Current progress is tracked in [3]_
-
-Current State / Anticipated Impact
-==================================
-
-There are over 100 repositories needing this change, a short selection is
-listed below.
-
-* openstack/ironic
-* openstack/ironic
-* openstack/ironic-ui
-* openstack/karbor-dashboard
-* openstack/keystoneauth
-* openstack/keystoneauth
-* openstack/keystoneauth
-* openstack/keystonemiddleware
-* openstack/keystonemiddleware
-* openstack/kuryr-tempest-plugin
-* openstack/magnum
-* openstack/manila
-* openstack/mistral-dashboard
-* openstack/mistral-tempest-plugin
-* openstack/monasca-analytics
-* openstack/monasca-analytics
-* openstack/monasca-ceilometer
-* openstack/monasca-events-api
-* openstack/monasca-log-api
-* openstack/monasca-statsd
-* openstack/monasca-tempest-plugin
-* openstack/monasca-transform
-* openstack/monasca-transform
-* openstack/monasca-ui
-* openstack/networking-l2gw
-* openstack/networking-l2gw-tempest-plugin
-* openstack/networking-midonet
-* openstack/networking-powervm
-* openstack/networking-sfc
-* openstack/neutron
-* openstack/neutron-dynamic-routing
-* openstack/neutron-fwaas
-* openstack/neutron-fwaas-dashboard
-* openstack/neutron-vpnaas-dashboard
-* openstack/nova
-* openstack/nova-powervm
-* openstack/openstack-ansible
-* openstack/openstack-doc-tools
-* openstack/openstack-doc-tools
-* openstack/openstack-health
-* openstack/openstacksdk
-* openstack/os-brick
-* openstack/os-collect-config
-
-
-Links
-=====
-
-.. [1] https://docs.python.org/3/library/unittest.mock.html
-.. [2] https://review.opendev.org/#/c/720914/
-.. [3] https://review.opendev.org/#/q/branch:master+topic:unittest.mock