Internal Remove GenericStringArray::from_vec and GenericStringArray::from_opt_vec

[alamb]

Which issue does this PR close?

Minor change

Rationale for this change

Less code is easier to maintain and faster to compile

I noticed this while working on #1145 / #197. These two functions are totally redundant, and are not part of the public api (are pub(crate))

What changes are included in this PR?

1. remove named functions (including one unsafe usage) and update callsites

Are there any user-facing changes?

1. No all functions are pub(crate)

[codecov-commenter]

Codecov Report

Merging #1147 (e88e526) into master (719096b) will decrease coverage by 0.01%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #1147      +/-   ##
==========================================
- Coverage   82.55%   82.54%   -0.02%     
==========================================
  Files         169      169              
  Lines       50456    50436      -20     
==========================================
- Hits        41655    41632      -23     
- Misses       8801     8804       +3     

Impacted Files	Coverage Δ
arrow/src/array/array_string.rs	97.29% <66.66%> (+0.21%)	⬆️
arrow/src/array/equal/mod.rs	93.11% <100.00%> (-0.03%)	⬇️
parquet_derive/src/parquet_field.rs	65.75% <0.00%> (-0.69%)	⬇️
parquet/src/encodings/encoding.rs	93.52% <0.00%> (-0.20%)	⬇️

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[jhorstmann]

Less code is nice 🚀