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dockershim: fine-tune network-ready handling on sandbox teardown and removal

If sandbox teardown results in an error, GC will periodically attempt
to again remove the sandbox.  Until the sandbox is removed, pod
sandbox status calls will attempt to enter the pod's namespace and
retrieve the pod IP, but the first teardown attempt may have already
removed the network namespace, resulting in a pointless log error
message that the network namespace doesn't exist, or that nsenter
can't find eth0.

The network-ready mechanism originally attempted to suppress those
messages by ensuring that pod sandbox status skipped network checks
when networking was already torn down, but unfortunately the ready
value was cleared too early.

Also, don't tear down the pod network multiple times if the first
time we tore it down, it succeeded.



**What this PR does / why we need it**:

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #

**Special notes for your reviewer**:

**Release note**:

```release-note
```