## What changes were proposed in this pull request?
Several cleanups in `ColumnarBatch`
* remove `schema`. The `ColumnVector`s inside `ColumnarBatch` already have the data type information, we don't need this `schema`.
* remove `capacity`. `ColumnarBatch` is just a wrapper of `ColumnVector`s, not builders, it doesn't need a capacity property.
* remove `DEFAULT_BATCH_SIZE`. As a wrapper, `ColumnarBatch` can't decide the batch size, it should be decided by the reader, e.g. parquet reader, orc reader, cached table reader. The default batch size should also be defined by the reader.
## How was this patch tested?
existing tests.
Author: Wenchen Fan <wenchen@databricks.com>
Closes#20316 from cloud-fan/columnar-batch.
d8aaa771e2