Remove destroy() methods.

Go does not automatically set destroy() methods as finalizers and no
code in gozmq calls runtime.SetFinalizer, nor does any code in gozmq
ever call one of the destroy() methods.
Run `go fmt *.go`.
Merge pull request #64 from jtacoma/remove-destroy

Remove destroy() methods.
Merge pull request #65 from jtacoma/go-fmt

Run `go fmt *.go`.