Fix web docs search function not working.

Remove obsolete html_theme_path setting from doc/walt/doc/sphinx/conf.py.
This was causing the search function to fail, as described here:
readthedocs/sphinx_rtd_theme#1434

doc/walt/doc/sphinx/conf.py

@@ -76,7 +76,6 @@ def get_version():
 # https://www.sphinx-doc.org/en/master/usage/configuration.html#options-for-html-output
 
 html_theme = "sphinx_rtd_theme"
-html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
 html_static_path = ["_static"]
 html_logo = "logo-walt.png"
 html_theme_options = {