Remove timer debug log from apicast logs

When running APIcast with APICAST_LOG_LEVEL=debug there are a ton of the following messages:

```
2019/10/22 17:46:49 [debug] 34#34: worker cycle
2019/10/22 17:46:49 [debug] 34#34: epoll timer: -1
2019/10/22 17:46:49 [debug] 34#34: timer signal
2019/10/22 17:46:49 [debug] 34#34: timer delta: 100
2019/10/22 17:46:49 [debug] 34#34: worker cycle
2019/10/22 17:46:49 [debug] 34#34: epoll timer: -1
2019/10/22 17:46:49 [debug] 34#34: timer signal
2019/10/22 17:46:49 [debug] 34#34: timer delta: 101
2019/10/22 17:46:49 [debug] 34#34: worker cycle
2019/10/22 17:46:49 [debug] 34#34: epoll timer: -1
2019/10/22 17:46:49 [debug] 34#34: timer signal
2019/10/22 17:46:49 [debug] 34#34: timer delta: 99
2019/10/22 17:46:49 [debug] 34#34: worker cycle
2019/10/22 17:46:49 [debug] 34#34: epoll timer: -1
2019/10/22 17:46:49 [debug] 34#34: timer signal
```

It seems to output several times per second. I'm not sure what the purpose of this log is as it wasn't present in previous versions. However, this adds a significant amount of noise and support will often ask customers to put apicast in debug mode while we troubleshoot and there are a lot of useful logs at the same "debug" level that we need.