Remove VCS dependency on Furo due to packaging issue (#331)

I didn't realize PyPI bans `install_requires` having a VCS (version
control system) requirement. That's a good ban, but it broke the
1.12.0rc1 release.

The workaround is to remove Furo from our `setup.py`. You don't need
Furo installed to use our normal Pytorch theme, only to use the
experiment `_qiskit_furo` theme. The docs still build even without Furo
thanks to conditional imports:


https://github.com/Qiskit/qiskit_sphinx_theme/blob/770d3b658a172d1b6e075e253901b8e141aac46e/qiskit_sphinx_theme/__init__.py#L63-L70

If you do want to use Furo, then you can still install it manually by
e.g. adding it to your `requirements-dev.txt`. All our theme cares about
is that Furo is in the virtual environment. It doesn't care how it got
there.

Once Furo has a stable release, we will move the requirement back to
setup.py. Tracked by
#330.

requirements-dev.txt

@@ -6,3 +6,5 @@ sphinx-design
 sphinx-autodoc-typehints
 nbsphinx==0.9.*
 pytest==7.*
+# Move to setup.py once they do a stable release.
+furo @ git+https://github.com/pradyunsg/furo.git@48c0bf2aa983fb1ccd79b4167fe85387249afedf

setup.py

@@ -30,7 +30,6 @@
     ],
     install_requires=[
         "docutils",
-        "furo @ git+https://github.com/pradyunsg/furo.git@48c0bf2aa983fb1ccd79b4167fe85387249afedf",
         "sphinx",
         "sphinxcontrib-jquery",  # Remove once we get rid of the Pytorch theme.
     ],