doc/conf: drop obsolete html_theme_path

Since sphinx_rtd_theme 3.0.0rc1, "html_theme_path" is obsolete:

  "Raise a warning when defining html_theme_path. This was an old config
  that’s not required anymore." [1]

Fixes warnings such as:

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

Fixes labgrid's CI which is building the docs with -W, treating warnings
as errors.

[1] https://sphinx-rtd-theme.readthedocs.io/en/stable/changelog.html

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

doc/conf.py

@@ -95,9 +95,6 @@
 #
 html_theme = 'sphinx_rtd_theme'
 
-# Set correct html_path for rtd theme:
-html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
-
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the
 # documentation.