Remove PrimitiveBuilder::finish_dict (#1978)

[tustvold]

Which issue does this PR close?

Closes #1978
Part of #1979

Rationale for this change

The finish_dict method has always struck me as a little bit strange, and it is definitely not safe. This PR just removes it

What changes are included in this PR?

Starts the process of #1979 and uses this to remove the need for PrimitiveBuilder::finish_dict

Are there any user-facing changes?

Yes, this removes an API

[codecov-commenter]

Codecov Report

Merging #1980 (954916d) into master (c47d14c) will increase coverage by 0.00%.
The diff coverage is 94.28%.

❗ Current head 954916d differs from pull request most recent head 85d9229. Consider uploading reports for the commit 85d9229 to get more accurate results

@@           Coverage Diff           @@
##           master    #1980   +/-   ##
=======================================
  Coverage   83.54%   83.54%           
=======================================
  Files         221      221           
  Lines       57395    57408   +13     
=======================================
+ Hits        47950    47961   +11     
- Misses       9445     9447    +2     

Impacted Files	Coverage Δ
arrow/src/array/builder/primitive_builder.rs	93.11% <ø> (+0.32%)	⬆️
.../src/array/builder/primitive_dictionary_builder.rs	78.68% <87.50%> (+0.50%)	⬆️
...row/src/array/builder/string_dictionary_builder.rs	90.64% <87.50%> (-1.09%)	⬇️
arrow/src/array/array_primitive.rs	95.01% <100.00%> (+0.03%)	⬆️
arrow/src/array/array_string.rs	97.76% <100.00%> (+0.03%)	⬆️
arrow/src/array/data.rs	84.54% <100.00%> (+0.13%)	⬆️
arrow/src/datatypes/datatype.rs	65.42% <0.00%> (-0.38%)	⬇️
parquet_derive/src/parquet_field.rs	65.98% <0.00%> (ø)

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[viirya]

This looks like an API should not public.