Clear tile cache on (re-)init.

Clearing the cache was disabled to avoid scenery reloading on sim resets,
which is now avoided elsewhere (in FG).
Cache cleaning is now needed to support new option for complete scenery
reloading.

simgear/scene/tgdb/TileCache.cxx

@@ -66,9 +66,7 @@ void TileCache::init( void ) {
     SG_LOG( SG_TERRAIN, SG_INFO, "  current cache size = "
             << tile_cache.size() );
 
-#if 0 // don't clear the cache
     clear_cache();
-#endif
 
     SG_LOG( SG_TERRAIN, SG_INFO, "  done with init()"  );
 }