[Polly] Remove unused declaration. NFC.

GitOrigin-RevId: f0f5afc4dd1fd753aea89ae64b508d29b94c092d
diff --git a/lib/Transform/ScheduleOptimizer.cpp b/lib/Transform/ScheduleOptimizer.cpp
index 9849f84..95a881e 100644
--- a/lib/Transform/ScheduleOptimizer.cpp
+++ b/lib/Transform/ScheduleOptimizer.cpp
@@ -338,8 +338,6 @@
   int k = -1;
 };
 
-extern bool DisablePollyTiling;
-
 class ScheduleTreeOptimizer {
 public:
   /// Apply schedule tree transformations.