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MINOR: [C++][Parquet] Remove unused SerializedPageReader::current_page_ (#45125)

### Rationale for this change

std::shared_ptr<Page> current_page_ has been defined but not used in the SerializedPageReader.

### What changes are included in this PR?

Simply remove it.

### Are these changes tested?

Pass CI.

### Are there any user-facing changes?

No

Authored-by: Gang Wu <ustcwg@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>