implemented deleteLabel

Author: Shubh Bapna

extended/src/main/java/io/kubernetes/client/extended/kubectl/KubectlLabel.java

@@ -37,6 +37,11 @@ public KubectlLabel<ApiType> addLabel(String key, String value) {
     return this;
   }
 
+  public KubectlLabel<ApiType> deleteLabel(String key) {
+    this.addingLabels.put(key, null);
+    return this;
+  }
+
   @Override
   public ApiType execute() throws KubectlException {
     verifyArguments();