Removing unused refcount variable

The rebranch branch is failing to build due to an unused refcount
variable in runtime.c. The variable is only used by an os_assert and
nowhere else. I've removed it and instead put the check directly in the
assert.

Author: etcwilde

src/BlocksRuntime/runtime.c

@@ -468,18 +468,16 @@ static void _Block_byref_assign_copy(void *dest, const void *arg, const int flag
 // Old compiler SPI
 static void _Block_byref_release(const void *arg) {
     struct Block_byref *byref = (struct Block_byref *)arg;
-    int32_t refcount;
 
     // dereference the forwarding pointer since the compiler isn't doing this anymore (ever?)
     byref = byref->forwarding;
-    
+
     // To support C++ destructors under GC we arrange for there to be a finalizer for this
     // by using an isa that directs the code to a finalizer that calls the byref_destroy method.
     if ((byref->flags & BLOCK_BYREF_NEEDS_FREE) == 0) {
         return; // stack or GC or global
     }
-    refcount = byref->flags & BLOCK_REFCOUNT_MASK;
-	os_assert(refcount);
+    os_assert(byref->flags & BLOCK_REFCOUNT_MASK);
     if (latching_decr_int_should_deallocate(&byref->flags)) {
         if (byref->flags & BLOCK_BYREF_HAS_COPY_DISPOSE) {
             struct Block_byref_2 *byref2 = (struct Block_byref_2 *)(byref+1);