[Fix](flexible partial update) Don't set default value for `TOlapTabl…

…eIndexSchema.unique_key_update_mode` (apache#43090)

Problem Summary:

If we explicitly set default value for field `unique_key_update_mode` in
`.thirft` file, `__isset.unique_key_update_mode` will always be `true`.
So `OlapTableSchemaParam::init_unique_key_update_mode` will fail to set
`_unique_key_update_mode` correctly if it receive a request from old
version FE.

gensrc/thrift/Descriptors.thrift

@@ -256,7 +256,7 @@ struct TOlapTableSchemaParam {
     11: optional string auto_increment_column
     12: optional i32 auto_increment_column_unique_id = -1
     13: optional Types.TInvertedIndexFileStorageFormat inverted_index_file_storage_format = Types.TInvertedIndexFileStorageFormat.V1
-    14: optional Types.TUniqueKeyUpdateMode unique_key_update_mode = Types.TUniqueKeyUpdateMode.UPSERT
+    14: optional Types.TUniqueKeyUpdateMode unique_key_update_mode
     15: optional i32 sequence_map_col_unique_id = -1
 }