unpin flatbuffers dependency version

[Cheappie]

Which issue does this PR close?

Closes #1667

[codecov-commenter]

Codecov Report

Merging #1668 (e093656) into master (e0a527b) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1668      +/-   ##
==========================================
- Coverage   83.12%   83.11%   -0.01%     
==========================================
  Files         193      193              
  Lines       55850    55850              
==========================================
- Hits        46426    46422       -4     
- Misses       9424     9428       +4     

Impacted Files	Coverage Δ
arrow/src/datatypes/datatype.rs	66.40% <0.00%> (-0.40%)	⬇️
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (-0.23%)	⬇️
parquet/src/encodings/encoding.rs	93.37% <0.00%> (-0.19%)	⬇️
arrow/src/array/transform/mod.rs	86.57% <0.00%> (-0.12%)	⬇️

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[tustvold]

The original change was made in #239 and doesn't appear to be relevant anymore. Thanks 👍