PR: Skip unregistering comm with destructor.

[impact27]

The comm can not still be registered when __del__ is being called:

• if __del__ is called, the reference count is at 0.
• if the reference count is at 0, then the comm manager doesn't have a reference to the comm.

This avoids spyder-ide/spyder-kernels#145

[ccordoba12]

@blink1073, could you take a look at this one? We need it for the next major release of Spyder, which we're trying to get out of the door in a month or so.

[blink1073]

LGTM, thanks!

[ccordoba12]

Thanks a lot @blink1073!