Fix unused variable error/warning in page_data.cu

[davidwendt]

Description

Fixes a minor compile error/warnings for unused variables in the cpp/src/io/parquet/page_data.cu source file.

cudf/cpp/src/io/parquet/page_data.cu -o CMakeFiles/cudf.dir/src/io/parquet/page_data.cu.o
/cudf/cpp/src/io/parquet/page_data.cu(636): error #177-D: parameter "s" was declared but never referenced

/cudf/cpp/src/io/parquet/page_data.cu(343): error #177-D: parameter "sb" was declared but never referenced
          detected during instantiation of "cuda::std::__4::pair<int, int> cudf::io::parquet::gpu::<unnamed>::gpuDecodeDictionaryIndices<sizes_only>(volatile cudf::io::parquet::gpu::<unnamed>::page_state_s *, volatile cudf::io::parquet::gpu::<unnamed>::page_state_buffers_s *, int, int) [with sizes_only=true]" 
(1720): here

/cudf/cpp/src/io/parquet/page_data.cu(527): error #177-D: parameter "sb" was declared but never referenced
          detected during instantiation of "cudf::size_type cudf::io::parquet::gpu::<unnamed>::gpuInitStringDescriptors<sizes_only>(volatile cudf::io::parquet::gpu::<unnamed>::page_state_s *, volatile cudf::io::parquet::gpu::<unnamed>::page_state_buffers_s *, int, int) [with sizes_only=true]" 
(1724): here

3 errors detected in the compilation of "/cudf/cpp/src/io/parquet/page_data.cu".

Found these with a Debug build using nvcc 11.5 and gcc 9.5.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[davidwendt]

/merge