get_html_theme_path is deprecated

[rly]

When generating docs for extensions or nwb/hdmf schema, we get this warning:

WARNING: Calling get_html_theme_path is deprecated. If you are calling it to define html_theme_path, you are safe to remove that code.

As long as we keep the html_theme variable, we can remove html_theme_path in

hdmf-docutils/hdmf_docutils/init_sphinx_extension_doc.py

Lines 400 to 401 in 98f23f3

	html_theme = "sphinx_rtd_theme"
	html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]