…tor (netty#10890)

Motivation:

A race detector discovered a data race in GlobalEventExecutor present in
netty 4.1.51.Final:

```
  Write of size 4 at 0x0000cea08774 by thread T103:
    #0 io.netty.util.internal.DefaultPriorityQueue.poll()Lio/netty/util/internal/PriorityQueueNode; DefaultPriorityQueue.java:113
    #1 io.netty.util.internal.DefaultPriorityQueue.poll()Ljava/lang/Object; DefaultPriorityQueue.java:31
    #2 java.util.AbstractQueue.remove()Ljava/lang/Object; AbstractQueue.java:113
    #3 io.netty.util.concurrent.AbstractScheduledEventExecutor.pollScheduledTask(J)Ljava/lang/Runnable; AbstractScheduledEventExecutor.java:133
    #4 io.netty.util.concurrent.GlobalEventExecutor.fetchFromScheduledTaskQueue()V GlobalEventExecutor.java:119
    #5 io.netty.util.concurrent.GlobalEventExecutor.takeTask()Ljava/lang/Runnable; GlobalEventExecutor.java:106
    #6 io.netty.util.concurrent.GlobalEventExecutor$TaskRunner.run()V GlobalEventExecutor.java:240
    #7 io.netty.util.internal.ThreadExecutorMap$2.run()V ThreadExecutorMap.java:74
    #8 io.netty.util.concurrent.FastThreadLocalRunnable.run()V FastThreadLocalRunnable.java:30
    #9 java.lang.Thread.run()V Thread.java:835
    #10 (Generated Stub) <null>

  Previous read of size 4 at 0x0000cea08774 by thread T110:
    #0 io.netty.util.internal.DefaultPriorityQueue.size()I DefaultPriorityQueue.java:46
    #1 io.netty.util.concurrent.GlobalEventExecutor$TaskRunner.run()V GlobalEventExecutor.java:263
    #2 io.netty.util.internal.ThreadExecutorMap$2.run()V ThreadExecutorMap.java:74
    #3 io.netty.util.concurrent.FastThreadLocalRunnable.run()V FastThreadLocalRunnable.java:30
    #4 java.lang.Thread.run()V Thread.java:835
    #5 (Generated Stub) <null>
```

The race is legit, but benign. To trigger it requires a TaskRunner to
begin exiting and set 'started' to false, more work to be scheduled
which starts a new TaskRunner, that work then needs to schedule
additional work which modifies 'scheduledTaskQueue', and then the
original TaskRunner checks 'scheduledTaskQueue'. But there is no danger
to this race as it can only produce a false negative in the condition
which causes the code to CAS 'started' which is thread-safe.

Modifications:

Delete problematic references to scheduledTaskQueue. The only way
scheduledTaskQueue could be modified since the last check is if another
TaskRunner is running, in which case the current TaskRunner doesn't
care.

Result:

Data-race free code, and a bit less code to boot.