GUI: Remove the parent from the button_idle DrawData

Removing it does not result in any visible difference. It was likeliy
set by mistake.

gui/ThemeEngine.cpp

@@ -227,7 +227,7 @@ static const DrawDataInfo kDrawDataDefaults[] = {
 	{kDDWidgetBackgroundEditText,   "widget_textedit",  true,   kDDNone},
 	{kDDWidgetBackgroundSlider,     "widget_slider",    true,   kDDNone},
 
-	{kDDButtonIdle,                 "button_idle",      true,   kDDWidgetBackgroundSlider},
+	{kDDButtonIdle,                 "button_idle",      true,   kDDNone},
 	{kDDButtonHover,                "button_hover",     false,  kDDButtonIdle},
 	{kDDButtonDisabled,             "button_disabled",  true,   kDDNone},
 	{kDDButtonPressed,              "button_pressed",   false,  kDDButtonIdle},