Remove enumerators for stale GC reasons.

Change-Id: I297e3ce5ba75e308cc994a7303d6bfc70e819737
diff --git a/vm/alloc/Heap.h b/vm/alloc/Heap.h
index 0d47e4b..faa967a 100644
--- a/vm/alloc/Heap.h
+++ b/vm/alloc/Heap.h
@@ -72,10 +72,6 @@
     GC_CONCURRENT,
     /* Explicit GC via Runtime.gc(), VMRuntime.gc(), or SIGUSR1. */
     GC_EXPLICIT,
-    /* GC to try to reduce heap footprint to allow more non-GC'ed memory. */
-    GC_EXTERNAL_ALLOC,
-    /* GC to dump heap contents to a file, only used under WITH_HPROF */
-    GC_HPROF_DUMP_HEAP
 } GcReason;
 
 /*