Scene: Remove unused `mesh_materials` StringNames

They were added in 8be2fabbe5cd846bac5e5a38e55f3fb70e73f2da (2.1 era) but
were likely a first attempt that didn't get unused in the end.

(cherry picked from commit 764eee03a438c81ec3a9f28f50201dcd77af96df)

scene/scene_string_names.cpp

@@ -195,11 +195,6 @@ SceneStringNames::SceneStringNames() {
 
 	_default = StaticCString::create("default");
 
-	for (int i = 0; i < MAX_MATERIALS; i++) {
-
-		mesh_materials[i] = "material/" + itos(i);
-	}
-
 	_mesh_changed = StaticCString::create("_mesh_changed");
 
 	parameters_base_path = "parameters/";

scene/scene_string_names.h

@@ -207,10 +207,6 @@ public:
 
 	StringName tracks_changed;
 
-	enum {
-		MAX_MATERIALS = 32
-	};
-	StringName mesh_materials[MAX_MATERIALS];
 	StringName _mesh_changed;
 };