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 <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: wgtmac

cpp/src/parquet/column_reader.cc

@@ -291,7 +291,6 @@ class SerializedPageReader : public PageReader {
   std::shared_ptr<ArrowInputStream> stream_;
 
   format::PageHeader current_page_header_;
-  std::shared_ptr<Page> current_page_;
 
   // Compression codec to use.
   std::unique_ptr<::arrow::util::Codec> decompressor_;