Not in use at the moment, was causing a compiler error on
Apple LLVM version 8.0.0 (clang-800.0.42.1).
```
src/mbgl/sprite/sprite_loader.cpp:25:11: error: call to constructor of
'Actor<mbgl::SpriteLoaderWorker>' is ambiguous
```
Otherwise, an ActorRef that's in the process of sending a message could attempt to access an invalid Scheduler reference:
Thread 1 Thread 2
--------------------------------------------------
Scheduler::Scheduler
Actor::Actor
weakMailbox.lock()
Actor::~Actor
Scheduler::~Scheduler
mailbox->push()
scheduler.schedule() 💣
