Postpone tacker-studio for train

- As no progress on code side in stein cycle, so moving tacker-studio.rst
for train cycle.
- Remove placeholder spec.

Co-Authored-By: Nguyen Hai <nguyentrihai93@gmail.com>
Change-Id: I8054f7db90eabd5ca09273f37ae6389ccc086705

specs/train/placeholder.rst

@@ -1,124 +0,0 @@
-..
- This work is licensed under a Creative Commons Attribution 3.0 Unported
- License.
-
- http://creativecommons.org/licenses/by/3.0/legalcode
-
-==================
-(Placeholder Spec)
-==================
-
-This file is just a placeholder for the specs directory.
-It will be removed soon after some spec is merged.
-
-The latest spec template is found at ``specs/template.rst``
-in the ``tacker-specs`` repository.
-
-Problem description
-===================
-
-Sphinx toctree complains if no file exists in a directory
-specified in toctree glob.
-
-Use Cases
----------
-
-None
-
-Proposed change
-===============
-
-Add this file. Please remove later when the another spec is approved.
-
-Alternatives
-------------
-
-None
-
-Data model impact
------------------
-
-None
-
-REST API impact
----------------
-
-None
-
-Security impact
----------------
-
-None
-
-Notifications impact
---------------------
-
-None
-
-Other end user impact
----------------------
-
-None
-
-Performance Impact
-------------------
-
-None
-
-Other deployer impact
----------------------
-
-None
-
-Developer impact
-----------------
-
-None
-
-Upgrade impact
---------------
-
-None
-
-
-Implementation
-==============
-
-Assignee(s)
------------
-
-None
-
-Work Items
-----------
-
-None
-
-
-Dependencies
-============
-
-None
-
-
-Testing
-=======
-
-None
-
-
-Documentation Impact
-====================
-
-None
-
-References
-==========
-
-None
-
-
-History
-=======
-
-None