doc: Remove obsolete files

modules/bullet/doc_classes/BulletPhysicsServer.xml

@@ -1,13 +0,0 @@
-<?xml version="1.0" encoding="UTF-8" ?>
-<class name="BulletPhysicsServer" inherits="PhysicsServer" version="3.2">
-	<brief_description>
-	</brief_description>
-	<description>
-	</description>
-	<tutorials>
-	</tutorials>
-	<methods>
-	</methods>
-	<constants>
-	</constants>
-</class>

modules/gridmap/doc_classes/README.md

@@ -1 +0,0 @@
-Doc classes will appear here when generating