remove stale comment on ArrayData equality and update unit tests

[jimexist]

Which issue does this PR close?

Closes #.

Rationale for this change

remove stale comment and update unit tests, because equality for ArrayData is implemented

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #472 (7c5beb9) into master (60fedf7) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 7c5beb9 differs from pull request most recent head 1b83439. Consider uploading reports for the commit 1b83439 to get more accurate results

@@            Coverage Diff             @@
##           master     #472      +/-   ##
==========================================
- Coverage   82.66%   82.65%   -0.01%     
==========================================
  Files         165      165              
  Lines       45550    45524      -26     
==========================================
- Hits        37653    37628      -25     
+ Misses       7897     7896       -1     

Impacted Files	Coverage Δ
arrow/src/array/array_struct.rs	87.84% <100.00%> (-0.59%)	⬇️
arrow/src/array/builder.rs	85.78% <100.00%> (-0.13%)	⬇️
parquet/src/encodings/encoding.rs	95.04% <0.00%> (+0.19%)	⬆️

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[nevi-me]

We can merge this after a rebase

[alamb]

👍