Fix doc build

Fix the following that shows up when running "python setup.py
build_sphinx":
manila/doc/source/adminref/index.rst:16:
WARNING: toctree contains reference to nonexisting document
u'adminref/launchpad'
manila/doc/source/adminref/index.rst:16:
WARNING: toctree contains reference to nonexisting document
u'adminref/gerrit'
manila/doc/source/adminref/index.rst:16:
WARNING: toctree contains reference to nonexisting document
u'adminref/jenkins'

checking consistency...
manila/doc/source/adminref/index.rst::
WARNING: document isn't included in any toctree

Cleanup adminref/index.rst file and remove content that is duplicated
from the devref/index.rst and not relevant here.

Change-Id: I57accea0f041ac59e507e4cf6461d2239f60b3b7