Remove dead bedrockDepth fields from generator

yes, this is not implemented yet. fields for this can be added when proper bedrock generation is implemented. For now, it's just dead code.

src/pocketmine/level/generator/hell/Nether.php

@@ -44,8 +44,6 @@ class Nether extends Generator{
 	private $emptyAmplitude = 1;
 	/** @var float */
 	private $density = 0.5;
-	/** @var int */
-	private $bedrockDepth = 5;
 
 	/** @var Populator[] */
 	private $generationPopulators = [];

src/pocketmine/level/generator/normal/Normal.php

@@ -44,8 +44,6 @@ class Normal extends Generator{
 	private $populators = [];
 	/** @var int */
 	private $waterHeight = 62;
-	/** @var int */
-	private $bedrockDepth = 5;
 
 	/** @var Populator[] */
 	private $generationPopulators = [];