Remove Depends on golang-github-spf13-viper-dev

from golang-github-spf13-cobra-dev in order to avoid a
"circular dependency hell" with golang-github-spf13-viper-dev,
golang-github-xordataexchange-crypt-dev, golang-etcd-server-dev
and finally golang-github-spf13-cobra-dev itself.

Thanks to Shengjing Zhu for the observation that the cobra library
does not actually need viper; only the cobra executable does.
(Closes: #869989)