BUG: Remove unused pydata-sphinx-theme docs dep

We now use furo.

Addresses:

Extension error (pydata_sphinx_theme.pygment):
Handler <function overwrite_pygments_css at 0x7fb8ff18c790> for event 'build-finished' threw an exception (exception: 'pygment_light_style')
m

docs/requirements.txt

@@ -2,7 +2,6 @@ furo
 imjoy_jupyterlab_extension
 jupyterlite[all]==0.1.0b17
 myst-parser[linkify]
-pydata-sphinx-theme
 sphinx-autodoc2>=0.5.0
 sphinx-copybutton
 sphinx-design