bpo-36396: Remove fgBg param of idlelib.config.GetHighlight() (GH-12491)

This param was only used once and changed the return type.
diff --git a/Lib/idlelib/configdialog.py b/Lib/idlelib/configdialog.py
index 5fdaf82..31520a3 100644
--- a/Lib/idlelib/configdialog.py
+++ b/Lib/idlelib/configdialog.py
@@ -1252,7 +1252,7 @@
             colors = idleConf.GetHighlight(theme, element)
             if element == 'cursor':  # Cursor sample needs special painting.
                 colors['background'] = idleConf.GetHighlight(
-                        theme, 'normal', fgBg='bg')
+                        theme, 'normal')['background']
             # Handle any unsaved changes to this theme.
             if theme in changes['highlight']:
                 theme_dict = changes['highlight'][theme]