clean up more dead code

src/core/SkTaskGroup.h

Index: src/core/SkTaskGroup.h
diff --git a/src/core/SkTaskGroup.h b/src/core/SkTaskGroup.h
index e6c36651fd50772a67b417e2044447e6c120fcdb..0d86cdb82934fefa17274c4a99e714f7751aa8b0 100644
--- a/src/core/SkTaskGroup.h
+++ b/src/core/SkTaskGroup.h
@@ -14,8 +14,6 @@
 #include "SkAtomics.h"
 #include "SkTemplates.h"
 
-struct SkRunnable;
-
 class SkTaskGroup : SkNoncopyable {
 public:
     // Create one of these in main() to enable SkTaskGroups globally.
@@ -28,9 +26,6 @@ public:
     ~SkTaskGroup() { this->wait(); }
 
     // Add a task to this SkTaskGroup.  It will likely run on another thread.
-    // Neither add() method takes owership of any of its parameters.
-    void add(SkRunnable*);
-
     void add(std::function<void(void)> fn);
 
     // Add a batch of N tasks, all calling fn with different arguments.