Revert docs LaTeX preamble and max list depth

This reverts commits 2820, 4afd1 and f5be.

ReadTheDocs PDF builds fail with "! LaTeX Error: Too deeply nested.".
Two of these errors are fixed by adding the LaTeX preamble:

    \usepackage{enumitem}
    \setlistdepth{10}

But increasing the list depth to 1000 did not fix a third error.
Monkeypatching the definition of Verbatim (from sphinx.sty) in the
preamble also did not fix the error.

sphinx-doc/sphinx#777

Monkeypatch:
http://trac.sagemath.org/ticket/9107#comment:75
http://git.sagemath.org/sage.git/commit/?id=891c3fad654e89e6b96bcf8f79114f631c8b7bba

docs/conf.py

@@ -42,10 +42,6 @@
 html_domain_indices = False  # If True, see modindex_common_prefix.
 html_show_sourcelink = False
 
-latex_elements = {
-    "preamble": "\\usepackage{enumitem}\n\\setlistdepth{50}",
-}
-
 
 extensions = [
     "sphinx.ext.autodoc",