Remove core.search_sites

This ran haystack.autodiscover(), which was obsoleted by Haystack 2.0.0.
In addition, search_sites itself was removed by Haystack 2.0.0
(HAYSTACK_SITECONF).

Author: lukegb

docs/api/core.rst

@@ -21,11 +21,6 @@
 .. automodule:: readthedocs.core.models
     :members:
 
-:mod:`readthedocs.core.search_sites`
-------------------------------------
-.. automodule:: readthedocs.core.search_sites
-    :members:
-
 :mod:`readthedocs.core.views`
 -----------------------------
 .. automodule:: readthedocs.core.views