[C++][Parquet] Remove unused member null_slot_usage in struct LevelInfo #50678

Describe the bug, including details regarding any error messages, version, and platform.

#50271 (comment)

I made a mistake earlier at #50271 (comment). I only noticed that it was a non-internal header file, but failed to notice that the struct was within the parquet::internal namespace.

Component(s)

C++, Parquet