Redundant condition check in SafepointSynchronize::thread_not_running

`SafepointSynchronize::thread_not_running` starts by checking if the current state is `is_running`:

```
bool SafepointSynchronize::thread_not_running(ThreadSafepointState *cur_state) {
  if (!cur_state->is_running()) {
    return true;
  }
  ...
}
```

However, this has been asserted by the caller, `SafepointSynchronize::synchronize_threads`.

## call site 1

```
  for (; JavaThread *cur = jtiwh.next(); ) {
    assert(cur->safepoint_state()->is_running(), "Illegal initial state");
  }
  ...
  if (thread_not_running(cur_tss)) {
```

## call site 2

```
      assert(cur_tss->is_running(), "Illegal initial state");
      if (thread_not_running(cur_tss)) {
```

Therefore, `!cur_state->is_running()` is always false, and the `if` is just skipped.