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PARQUET-1467: [C++] Remove defunct ChunkedAllocator code

It does not seem that memory allocation on the dictionary encoding path requires something so elaborate right now

Author: Wes McKinney <wesm+git@apache.org>

Closes #3069 from wesm/PARQUET-1467 and squashes the following commits:

f37ed0756 <Wes McKinney> Remove defunct memory allocator code