Remove flatbuffers dependency

[jjacobelli]

As discussed with @jakirkham and @trxcllnt, cudf does not depend directly on flatbuffers so we can remove the dependency

[jjacobelli]

Integration PR: rapidsai/integration#309

[codecov]

Codecov Report

Merging #8671 (edeab2f) into branch-21.08 (7721819) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head edeab2f differs from pull request most recent head d338119. Consider uploading reports for the commit d338119 to get more accurate results

@@               Coverage Diff                @@
##           branch-21.08    #8671      +/-   ##
================================================
+ Coverage         10.62%   10.64%   +0.01%     
================================================
  Files               109      109              
  Lines             18289    18654     +365     
================================================
+ Hits               1943     1985      +42     
- Misses            16346    16669     +323     

Impacted Files	Coverage Δ
python/cudf/cudf/io/hdf.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/_version.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/abc.py	0.00% <0.00%> (ø)
python/cudf/cudf/api/types.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/dlpack.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/feather.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/parquet.py	0.00% <0.00%> (ø)
... and 44 more

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[jjacobelli]

@gpucibot merge