#840: Adjust annotations in Display entity interface

- Undeprecate #get/setGlowColorOverride() (reverts ff78a4cdaf, see f328857bac)
- Remove erroneous @NotNull annotation on setBillboard()

By: Parker Hawke <hawkeboyz2@hotmail.com>

paper-api/src/main/java/org/bukkit/entity/Display.java

@@ -143,26 +143,21 @@ public interface Display extends Entity {
      *
      * @param billboard new setting
      */
-    @NotNull
     public void setBillboard(@NotNull Billboard billboard);
 
     /**
      * Gets the scoreboard team overridden glow color of this display.
      *
      * @return glow color
-     * @deprecated API subject to change
      */
     @Nullable
-    @Deprecated
     public Color getGlowColorOverride();
 
     /**
      * Sets the scoreboard team overridden glow color of this display.
      *
      * @param color new color
-     * @deprecated API subject to change
      */
-    @Deprecated
     public void setGlowColorOverride(@Nullable Color color);
 
     /**