Revert "add waiters method for dispatcher and executor"

This reverts commit 3da4b37.

Author: Alexey Andreev

src/txapi.rs

@@ -73,10 +73,6 @@ impl Dispatcher {
     pub fn len(&self) -> usize {
         self.task_tx.len()
     }
-
-    pub fn waiters(&self) -> usize {
-        self.waiters.load(atomic::Ordering::Relaxed)
-    }
 }