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" comment has been superseded by Go modules.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".

Kubernetes-commit: 8a908e0c0bd96a3455edf7e3b5f5af90564e65b0

doc.go

@@ -14,4 +14,4 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-package kubectl // import "k8s.io/kubectl"
+package kubectl

pkg/cmd/set/env/doc.go

@@ -15,4 +15,4 @@ limitations under the License.
 */
 
 // Package env provides functions to incorporate environment variables into set env.
-package env // import "k8s.io/kubectl/pkg/cmd/set/env"
+package env

pkg/util/openapi/doc.go

@@ -18,4 +18,4 @@ limitations under the License.
 // from a Kubernetes server and then indexing the type definitions.
 // The openapi spec contains the object model definitions and extensions metadata
 // such as the patchStrategy and patchMergeKey for creating patches.
-package openapi // import "k8s.io/kubectl/pkg/util/openapi"
+package openapi