remove import doc comments

The "// import <path>" comment has been superseded by Go modules.
We don't have to remove them, but doing so has some advantages:

- They are used inconsistently, which is confusing.
- We can then also remove the (currently broken) hack/update-vanity-imports.sh.
- Last but not least, it would be a first step towards avoiding the k8s.io domain.

This commit was generated with
   sed -i -e 's;^package \(.*\) // import.*;package \1;' $(git grep -l '^package.*// import' | grep -v 'vendor/')

Everything was included, except for
   package labels // import k8s.io/kubernetes/pkg/util/labels
because that package is marked as "read-only".

pkg/proxy/doc.go

@@ -15,4 +15,4 @@ limitations under the License.
 */
 
 // Package proxy implements the layer-3 network proxy.
-package proxy // import "k8s.io/kubernetes/pkg/proxy"
+package proxy