integration: enable TestV3WatchRestoreSnapshotUnsync for gRPC proxy

Signed-off-by: Gyuho Lee <gyuhox@gmail.com>

integration/v3_watch_restore_test.go

@@ -12,8 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build !cluster_proxy
-
 package integration
 
 import (