Clean-up unused variables and prevent some unused variable warnings

[WardBrian]

Submission Checklist

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

Release notes

Removed some unused variables in the generated C++ code and prevented some additional -Wunused-variable warnings

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[bot]]

Codecov Report

Merging #1321 (68b91de) into master (f2a65c8) will decrease coverage by 0.01%. The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master    #1321      +/-   ##
==========================================
- Coverage   88.79%   88.79%   -0.01%     
==========================================
  Files          64       64              
  Lines        9853     9867      +14     
==========================================
+ Hits         8749     8761      +12     
- Misses       1104     1106       +2     

Impacted Files	Coverage Δ
src/stan_math_backend/Transform_Mir.ml	94.49% <81.25%> (-0.79%)	:arrow_down:
src/stan_math_backend/Cpp.ml	85.94% <100.00%> (+0.30%)	:arrow_up:
src/stan_math_backend/Lower_functions.ml	99.45% <100.00%> (ø)
src/stan_math_backend/Lower_program.ml	99.07% <100.00%> (+<0.01%)	:arrow_up:
src/stan_math_backend/Lower_stmt.ml	96.84% <100.00%> (ø)