Copter: tidy invocation of precland.update

Setting the altitude to home-relative is misleading/wasteful in this method as
the alt is unused in precland unless it is marked as "OK" with the
second parameter

ArduCopter/precision_landing.cpp

@@ -13,13 +13,8 @@ void Copter::init_precland()
 
 void Copter::update_precland()
 {
-    int32_t height_above_ground_cm = current_loc.alt;
-
-    // use range finder altitude if it is valid, otherwise use home alt
-    if (rangefinder_alt_ok()) {
-        height_above_ground_cm = rangefinder_state.alt_cm_glitch_protected;
-    }
-
-    precland.update(height_above_ground_cm, rangefinder_alt_ok());
+    // alt will be unused if we pass false through as the second parameter:
+    return precland.update(rangefinder_state.alt_cm_glitch_protected,
+                           rangefinder_alt_ok());
 }
 #endif