Move hotbar code/textures, and damage sound, from player_api to default

These are unrelated to the Player API and player object.
All other GUI/HUD code and textures are in default.
All other engine hardcoded sounds are in default.
The player_api mod is highly likely to be used unchanged in new games,
so logical grouping of content will help the creation of new games.

mods/player_api/init.lua

@@ -31,6 +31,4 @@ minetest.register_on_joinplayer(function(player)
 		{x = 200, y = 219},
 		30
 	)
-	player:hud_set_hotbar_image("gui_hotbar.png")
-	player:hud_set_hotbar_selected_image("gui_hotbar_selected.png")
 end)