removing upgrading.rst - this has already been moved into several documents in its own section

So, this is not really removing anything - it's just that this article was "moved" originally,
in #5186, but then later on a rebase, it appeared that this upgrading.rst was "added".

The result is the intention of the #5186 PR: to add a new "install" section with
*just* the new "unstable" article.

Author: weaverryan

cookbook/install/index.rst

@@ -4,5 +4,4 @@ Install and Upgrade
 .. toctree::
     :maxdepth: 2
 
-    upgrading
     unstable_versions

cookbook/install/upgrading.rst

@@ -116,7 +116,6 @@
 
 * :doc:`/cookbook/install/index`
 
-  * :doc:`/cookbook/install/upgrading`
   * :doc:`/cookbook/install/unstable_versions`
 
 * :doc:`/cookbook/logging/index`

cookbook/map.rst.inc

@@ -14,7 +14,6 @@
 /cookbook/service_container/parentservices /components/dependency_injection/parentservices
 /cookbook/service_container/factories /components/dependency_injection/factories
 /cookbook/service_container/tags /components/dependency_injection/tags
-/cookbook/upgrading /cookbook/install/upgrading
 /reference/configuration/mongodb /bundles/DoctrineMongoDBBundle/config
 /reference/YAML /components/yaml
 /components/dependency_injection /components/dependency_injection/introduction