scripts/release: stop using ETCDCTL_API=3

Note: v3 has been the default since 25bc657.

scripts/release

@@ -142,7 +142,7 @@ main() {
 
   # Sanity checks.
   ./release/etcd-${RELEASE_VERSION}-linux-amd64/etcd --version | grep -q "etcd Version: ${VERSION}"
-  ETCDCTL_API=3 ./release/etcd-${RELEASE_VERSION}-linux-amd64/etcdctl version | grep -q "etcdctl version: ${VERSION}"
+  ./release/etcd-${RELEASE_VERSION}-linux-amd64/etcdctl version | grep -q "etcdctl version: ${VERSION}"
 
   # Upload artifacts.
   if [ "${NO_UPLOAD}" == 1 ]; then
@@ -188,9 +188,9 @@ main() {
   # TODO: test
   # docker run --rm --name etcd-gcr-${RELEASE_VERSION} gcr.io/etcd-development/etcd:${RELEASE_VERSION};
   # docker exec etcd-gcr-${RELEASE_VERSION} /bin/sh -c "/usr/local/bin/etcd --version"
-  # docker exec etcd-gcr-${RELEASE_VERSION} /bin/sh -c "ETCDCTL_API=3 /usr/local/bin/etcdctl version"
-  # docker exec etcd-gcr-${RELEASE_VERSION} /bin/sh -c "ETCDCTL_API=3 /usr/local/bin/etcdctl put foo bar"
-  # docker exec etcd-gcr-${RELEASE_VERSION} /bin/sh -c "ETCDCTL_API=3 /usr/local/bin/etcdctl get foo"
+  # docker exec etcd-gcr-${RELEASE_VERSION} /bin/sh -c "/usr/local/bin/etcdctl version"
+  # docker exec etcd-gcr-${RELEASE_VERSION} /bin/sh -c "/usr/local/bin/etcdctl put foo bar"
+  # docker exec etcd-gcr-${RELEASE_VERSION} /bin/sh -c "/usr/local/bin/etcdctl get foo"
 
   # Bump version to next development version.
   git checkout -q "${BRANCH}" # Since we might be on a checkout of the remote version tag.