Revert "Selectively initialize SoA types"

[WardBrian]

Submission Checklist

• Run unit tests
• Documentation
  • If a user-facing facing change was made, the documentation PR is here:
  • OR, no user-facing changes were made

Release notes

Reverted #1323 as stan-dev/math#2978 provides a better solution at a lower level.

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to
license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov]

Codecov Report

Merging #1376 (a80ac26) into master (7b69a19) will decrease coverage by 0.01%.
Report is 5 commits behind head on master.
The diff coverage is 66.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1376      +/-   ##
==========================================
- Coverage   89.93%   89.93%   -0.01%     
==========================================
  Files          63       63              
  Lines       10704    10699       -5     
==========================================
- Hits         9627     9622       -5     
  Misses       1077     1077              

Files	Coverage Δ
src/analysis_and_optimization/Optimize.ml	93.23% <66.66%> (-0.07%)	⬇️

[WardBrian]

With the new tests added, https://jenkins.flatironinstitute.org/blue/organizations/jenkins/Stan%2FStanc3/detail/PR-1376/2/pipeline/ failed (as expected)

Running again against @SteveBronder's branch from stan-dev/math#2978: https://jenkins.flatironinstitute.org/blue/organizations/jenkins/Stan%2FStanc3/detail/PR-1376/3/pipeline