Corrected Instance timers when entering a map (#3594)

* Fixes #3585.
* Resolves an issue when players change maps to instances (aside from script command instance_enter) and the active/idle timers are not adjusted.
* Cleaned up the entering and leaving checks for instances.
Thanks to @Atemo and @Lemongrass3110!

src/map/instance.cpp

@@ -806,12 +806,6 @@ enum e_instance_enter instance_enter(struct map_session_data *sd, unsigned short
 	if (pc_setpos(sd, map_id2index(m), x, y, CLR_OUTSIGHT))
 		return IE_OTHER;
 
-	// If there was an idle timer, let's stop it
-	instance_stopidletimer(im, instance_id);
-
-	// Now we start the instance timer
-	instance_startkeeptimer(im, instance_id);
-
 	return IE_OK;
 }