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GH-38712: [Python] Remove dead code in _reconstruct_block (#38714)

### Rationale for this change

It seems the object case in `_reconstruct_block` is a dead code and is not needed anymore so therefore could be removed.

### What changes are included in this PR?

Removal of the object case in `_reconstruct_block` code. Was also looking at the `arrow_to_pandas.cc` code to see if there is any dead code present and I couldn't find any. 

### Are these changes tested?

The change in this PR should not make any of the existing tests fail.

### Are there any user-facing changes?

There shouldn't be.
* Closes: #38712

Authored-by: AlenkaF <frim.alenka@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>