Tabs: Remove unused 'panel' stylebox from default theme

Cf. https://github.com/godotengine/godot/issues/37875#issuecomment-625297308.

(cherry picked from commit cd8d3219619d041cb29f4cf1047f2e68eea4a8e4)

doc/classes/Tabs.xml

@@ -308,8 +308,6 @@
 		</theme_item>
 		<theme_item name="label_valign_fg" type="int" default="0">
 		</theme_item>
-		<theme_item name="panel" type="StyleBox">
-		</theme_item>
 		<theme_item name="tab_bg" type="StyleBox">
 			The style of an inactive tab.
 		</theme_item>

scene/resources/default_theme/default_theme.cpp

@@ -754,7 +754,6 @@ void fill_default_theme(Ref<Theme> &theme, const Ref<Font> &default_font, const
 	theme->set_stylebox("tab_fg", "Tabs", sb_expand(make_stylebox(tab_current_png, 4, 3, 4, 1, 16, 3, 16, 2), 2, 2, 2, 2));
 	theme->set_stylebox("tab_bg", "Tabs", sb_expand(make_stylebox(tab_behind_png, 5, 4, 5, 1, 16, 5, 16, 2), 3, 3, 3, 3));
 	theme->set_stylebox("tab_disabled", "Tabs", sb_expand(make_stylebox(tab_disabled_png, 5, 5, 5, 1, 16, 6, 16, 4), 3, 0, 3, 3));
-	theme->set_stylebox("panel", "Tabs", tc_sb);
 	theme->set_stylebox("button_pressed", "Tabs", make_stylebox(button_pressed_png, 4, 4, 4, 4));
 	theme->set_stylebox("button", "Tabs", make_stylebox(button_normal_png, 4, 4, 4, 4));