Remove redundant workaround for sphinx_rtd_theme

We no longer import sphinx_rtd_theme in doc/conf.py, so documentation
tests now run without requiring sphinx_rtd_theme to be installed.

doc/conf.py

@@ -6,9 +6,6 @@ import os
 # documentation root, use os.path.abspath to make it absolute, as shown here.
 sys.path.insert(0, os.path.abspath(os.path.join('..', 'src')))
 
-# Determine whether the documentation is being built on Read The Docs.
-on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
-
 # -- General configuration ------------------------------------------------
 
 # If your documentation needs a minimal Sphinx version, state it here.

noxfile.py

@@ -64,10 +64,6 @@ def tests(session):
         './tests',
         './doc',
         *session.posargs,
-        env={
-            # NOTE: Do not import sphinx_rtd_theme in doc/conf.py.
-            'READTHEDOCS': 'True',
-        },
     )
     # Ensure that regression test outputs have not changed.
     session.run(