DRTVWR-494: Remove LLMainThreadTask::dispatch(LockStatic&, ...)

Monty's code review reveals that conflating dispatch() with [un]lock
functionality is inconsistent and unnecessary.

indra/llcommon/llmainthreadtask.h

@@ -15,11 +15,9 @@
 
 #include "lleventtimer.h"
 #include "llthread.h"
-#include "lockstatic.h"
 #include "llmake.h"
 #include <future>
 #include <type_traits>              // std::result_of
-#include <boost/signals2/dummy_mutex.hpp>
 
 /**
  * LLMainThreadTask provides a way to perform some task specifically on the
@@ -42,33 +40,17 @@
  * will fulfill a future with its result. Meanwhile the requesting thread
  * blocks on that future. As soon as it is set, the requesting thread wakes up
  * with the task result.
- *
- * Under some circumstances it's necessary for the calling thread to hold a
- * lock until the task has been scheduled -- yet important to release the lock
- * while waiting for the result. If you pass a LockStatic<T> to dispatch(),
- * a secondary thread will unlock it before blocking on the future. (The main
- * thread simply holds the lock for the duration of the task.)
  */
 class LLMainThreadTask
 {
 private:
     // Don't instantiate this class -- use dispatch() instead.
     LLMainThreadTask() {}
-    // If our caller doesn't explicitly pass a LockStatic<something>, make a
-    // fake one.
-    struct Static
-    {
-        boost::signals2::dummy_mutex mMutex;
-    };
-    typedef llthread::LockStatic<Static> LockStatic;
 
 public:
     /// dispatch() is the only way to invoke this functionality.
-    /// If you call it with a LockStatic<something>, dispatch() unlocks it
-    /// before blocking for the result.
-    template <typename Static, typename CALLABLE>
-    static auto dispatch(llthread::LockStatic<Static>& lk, CALLABLE&& callable)
-        -> decltype(callable())
+    template <typename CALLABLE>
+    static auto dispatch(CALLABLE&& callable) -> decltype(callable())
     {
         if (on_main_thread())
         {
@@ -82,25 +64,12 @@ public:
             // Once we enable C++17, we can use Class Template Argument
             // Deduction. Until then, use llmake_heap().
             auto* task = llmake_heap<Task>(std::forward<CALLABLE>(callable));
-            // The moment we construct a new LLEventTimer subclass object, its
-            // tick() method might get called. However, its tick() method
-            // might depend on something locked by the passed LockStatic.
-            // Unlock it so tick() can proceed.
-            lk.unlock();
             auto future = task->mTask.get_future();
             // Now simply block on the future.
             return future.get();
         }
     }
 
-    /// You can call dispatch() without a LockStatic<something>.
-    template <typename CALLABLE>
-    static auto dispatch(CALLABLE&& callable) -> decltype(callable())
-    {
-        LockStatic lk;
-        return dispatch(lk, std::forward<CALLABLE>(callable));
-    }
-
 private:
     template <typename CALLABLE>
     struct Task: public LLEventTimer