Revert "Add sphinx_rtd_theme"

This is no longer required since the hook has been fixed
(see 768c658).

This reverts commit 3d2c63c.

tests/requirements-libraries.txt

@@ -14,7 +14,6 @@ pylint>=1.5
 markdown
 simplejson # simplejson is used for text_c_extension
 sphinx
-sphinx_rtd_theme
 pyzmq
 zope.interface  # Required for test_namespace_package
 # numpy 1.11 is broken and does not compile on AppVeyor (Windows)