doc: always use the RTD theme

readthedocs.org doesn't seem to use this as the default theme anymore.

doc/conf.py

@@ -22,8 +22,6 @@
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 #sys.path.insert(0, os.path.abspath('.'))
 
-on_rtd = os.getenv('READTHEDOCS') == 'True'
-
 # Read the source code file that contains the assembly info
 
 with open('../KeeAgent/Properties/AssemblyInfo.cs') as f:
@@ -121,12 +119,9 @@
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-if on_rtd:
-    html_theme = 'default'
-else:
-    import sphinx_rtd_theme
-    html_theme = 'sphinx_rtd_theme'
-    html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+import sphinx_rtd_theme
+html_theme = 'sphinx_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