added delete_dataset method (#1051)

### What problem does this PR solve?

Added delete_dataset method and test for it.

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

sdk/python/ragflow/ragflow.py

     def create_dataset(self, name):
         return name
 
+    def delete_dataset(self, name):
+        return name

sdk/python/test/test_basic.py

 
     def test_create_dataset(self):
         assert ragflow.ragflow.RAGFLow('123', 'url').create_dataset('abc') == 'abc'
+
+    def test_delete_dataset(self):
+        assert ragflow.ragflow.RAGFLow('123', 'url').delete_dataset('abc') == 'abc'