[Data] Remove buffer capacity from ShufflingBatcher (#48243)

## Why are these changes needed?

the ShufflingBatcher._buffer_capacity field is not used anywhere so we
can remove it.

python/ray/data/_internal/batcher.py

@@ -211,10 +211,6 @@ def __init__(
             # Round it up internally to `batch_size` since our algorithm requires it.
             # This is harmless since it only offers extra randomization.
             shuffle_buffer_min_size = batch_size
-        self._buffer_capacity = max(
-            2 * shuffle_buffer_min_size,
-            shuffle_buffer_min_size + batch_size,
-        )
         self._buffer_min_size = shuffle_buffer_min_size
         self._builder = DelegatingBlockBuilder()
         self._shuffle_buffer: Block = None