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.

README.txt

@@ -4,6 +4,7 @@ See license.txt for license information.
 
 Provides an API to allow multiple mods to set player models and textures.
 Also sets the default model, texture, and player flags.
+This mod is only for content related to the Player API and the player object.
 
 Authors of source code
 ----------------------
@@ -19,14 +20,6 @@ stujones11 (CC BY-SA 3.0):
 Jordach (CC BY-SA 3.0):
   character.png
 
-paramat (CC BY-SA 3.0):
-  gui_hotbar.png
-  gui_hotbar_selected.png
-
 celeron55, Perttu Ahola <celeron55@gmail.com> (CC BY-SA 3.0):
   player.png
   player_back.png
-
-sonictechtonic (CC BY 3.0):
-https://www.freesound.org/people/sonictechtonic/sounds/241872/
-  player_damage.ogg

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)

license.txt

@@ -22,7 +22,6 @@ Attribution-ShareAlike 3.0 Unported (CC BY-SA 3.0)
 Copyright (C) 2011-2018 celeron55, Perttu Ahola <celeron55@gmail.com>
 Copyright (C) 2012-2018 Jordach
 Copyright (C) 2018 stujones11
-Copyright (C) 2018 paramat
 
 You are free to:
 Share — copy and redistribute the material in any medium or format.
@@ -51,34 +50,3 @@ rights may limit how you use the material.
 
 For more details:
 http://creativecommons.org/licenses/by-sa/3.0/
-
-------------------------------------
-
-Attribution 3.0 Unported (CC BY 3.0)
-Copyright (C) 2014-2018 sonictechtonic
-
-You are free to:
-Share — copy and redistribute the material in any medium or format.
-Adapt — remix, transform, and build upon the material for any purpose, even commercially.
-The licensor cannot revoke these freedoms as long as you follow the license terms.
-
-Under the following terms:
-
-Attribution — You must give appropriate credit, provide a link to the license, and
-indicate if changes were made. You may do so in any reasonable manner, but not in any way
-that suggests the licensor endorses you or your use.
-
-No additional restrictions — You may not apply legal terms or technological measures that
-legally restrict others from doing anything the license permits.
-
-Notices:
-
-You do not have to comply with the license for elements of the material in the public
-domain or where your use is permitted by an applicable exception or limitation.
-No warranties are given. The license may not give you all of the permissions necessary
-for your intended use. For example, other rights such as publicity, privacy, or moral
-rights may limit how you use the material.
-
-For more details:
-http://creativecommons.org/licenses/by/3.0/
-