ArduCopter: stop passing mission in Terrain constructor

Terrain can use the mission singleton

This means Copter can have terrain while compiling mission out

ArduCopter/Copter.h

@@ -512,8 +512,8 @@ private:
 #endif
 
     // terrain handling
-#if AP_TERRAIN_AVAILABLE && MODE_AUTO_ENABLED == ENABLED
-    AP_Terrain terrain{mode_auto.mission};
+#if AP_TERRAIN_AVAILABLE
+    AP_Terrain terrain;
 #endif
 
     // Precision Landing

ArduCopter/config.h

@@ -664,10 +664,6 @@
   #error ModeAuto requires ModeRTL which is disabled
 #endif
 
-#if AP_TERRAIN_AVAILABLE && !MODE_AUTO_ENABLED
-  #error Terrain requires ModeAuto which is disabled
-#endif
-
 #if FRAME_CONFIG == HELI_FRAME && !MODE_ACRO_ENABLED
   #error Helicopter frame requires acro mode support which is disabled
 #endif