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

[wesm]

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

[codecov-io]

Codecov Report

Merging #3069 into master will increase coverage by 1.09%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3069      +/-   ##
==========================================
+ Coverage   87.09%   88.18%   +1.09%     
==========================================
  Files         489      431      -58     
  Lines       69254    65209    -4045     
==========================================
- Hits        60314    57504    -2810     
+ Misses       8839     7705    -1134     
+ Partials      101        0     -101

Impacted Files	Coverage Δ
cpp/src/parquet/column_writer.h	70.58% <ø> (ø)	⬆️
cpp/src/parquet/util/memory.h	100% <ø> (ø)	⬆️
cpp/src/parquet/util/memory-test.cc	100% <ø> (ø)	⬆️
cpp/src/parquet/util/memory.cc	86.45% <ø> (+0.11%)	⬆️
cpp/src/parquet/encoding-test.cc	100% <100%> (ø)	⬆️
cpp/src/parquet/test-util.h	95.05% <100%> (ø)	⬆️
cpp/src/parquet/encoding-internal.h	97.51% <100%> (-0.02%)	⬇️
cpp/src/parquet/column_writer.cc	95.88% <100%> (-0.01%)	⬇️
go/arrow/array/table.go
go/arrow/math/uint64_amd64.go
... and 56 more

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[wesm]

+1