Removing Alembic migration devref document

This patch removes unwanted devref documents w.r.t. alembic migration.
IMO, separate devref for sub-project is not required as it's already
documented @
http://docs.openstack.org/developer/neutron/devref/alembic_migrations.html#independent-sub-project-tables

Change-Id: Ida41846ff5d1be0058fd6881cceaa834c37268d7
Partial-Bug: #1560003

doc/source/others/alembic_migration.rst

@@ -1,28 +0,0 @@
-..
-      Copyright 2016 Huawei Technologies India Pvt Limited.
-
-      Licensed under the Apache License, Version 2.0 (the "License"); you may
-      not use this file except in compliance with the License. You may obtain
-      a copy of the License at
-
-          http://www.apache.org/licenses/LICENSE-2.0
-
-      Unless required by applicable law or agreed to in writing, software
-      distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
-      WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
-      License for the specific language governing permissions and limitations
-      under the License.
-
-
-      Convention for heading levels in Neutron devref:
-      =======  Heading 0 (reserved for the title in a document)
-      -------  Heading 1
-      ~~~~~~~  Heading 2
-      +++++++  Heading 3
-      '''''''  Heading 4
-      (Avoid deeper levels because they do not render well.)
-
-Alembic-migration
-=================
-
-TODO: Coming Soon

neutron_dynamic_routing/db/migration/README

@@ -1,2 +1,2 @@
 For details refer to:
-http://docs.openstack.org/developer/neutron-dynamic-routing/alembic_migration.html
+http://docs.openstack.org/developer/neutron/devref/alembic_migrations.html#independent-sub-project-tables