degrade "Warning: catching up on tile delete queue" from SG_ALERT to SG_WARN.

* it says it's a warning (while in fact it's just saying what it's doing)
* the user can't do much here (yes, flying slower, but it doesn't say that :-)
* scrolling those countless messages in the terminal puts stress on the CPU
  in a time when it's apparently already struggling

src/Scenery/tilemgr.cxx

@@ -363,7 +363,7 @@ void FGTileMgr::update_queues()
                 // get real serious and agressively free up some tiles so
                 // we don't explode our memory usage.
 
-                SG_LOG( SG_TERRAIN, SG_ALERT,
+                SG_LOG( SG_TERRAIN, SG_WARN,
                         "Warning: catching up on tile delete queue" );
             }