Remove workarounds for text documentation generation

[lpiwowar]

Remove workarounds. Some documentations do not work with the feature of sphinx-build that allows generation of the docs in text format.

List of issues:

• designate = with custom ext.support_matrix extension the generation of the documentation gets stuck in infinite loop
• ironic = with sphinxcontrib.apidoc extension the generation of the documentation gets stuck in infinite loop
• heat = AttributeError: 'TextTranslator' object has no attribute '_classifier_count_in_li' when doc/source/template_guide documentation is present
• trove/zaqar = The doc/requirements.txt file does not install all deps required to generate the docs

rag-content/scripts/get_openstack_plaintext_docs.sh

Line 140 in c74311e

# TODO(lpiwowar): Remove workarounds. Some of the documentations do not work with