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GH-41545: [C++][Parquet] Fix DeltaLengthByteArrayEncoder::EstimatedDataEncodedSize (#41546)



### Rationale for this change

`DeltaLengthByteArrayEncoder::EstimatedDataEncodedSize` would return an wrong estimate when `Put(const Array&)` was called.

### What changes are included in this PR?

Remove `encoded_size_` and uses `sink_.length()` as `encoded_size_`.

### Are these changes tested?

Yes

### Are there any user-facing changes?

No

* GitHub Issue: #41545

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>