Remove sphinx_autodoc_typehints

sphinx_autodoc_typehints has had some long-running bugs related
to the reST `container` directive.

The current bug is ...

    tox-dev/sphinx-autodoc-typehints#513

... which mangles the formatting of docstrings that have a return-type
annotation and a `container` directive.

This commit removes sphinx_autodoc_typethints. The extension doesn't
look like it was providing much value to the docs anyway.

docs/source/conf.py

@@ -14,7 +14,6 @@
     "sphinx.ext.intersphinx",
     "sphinx.ext.todo",
     "sphinx.ext.viewcode",
-    "sphinx_autodoc_typehints",
     "sphinx_toggleprompt",
     "uqbar.sphinx.api",
     "uqbar.sphinx.book",