Miscellaneous cleanups of protobuf generator plugin

[mjcarroll]

Removes some unused code and variables and fixes some style nits.

[codecov[bot]]

Codecov Report

Merging #405 (bb2aab4) into gz-msgs10 (8021595) will increase coverage by 0.40%. The diff coverage is 100.00%.

:exclamation: Current head bb2aab4 differs from pull request most recent head 645d5e4. Consider uploading reports for the commit 645d5e4 to get more accurate results

@@              Coverage Diff              @@
##           gz-msgs10     #405      +/-   ##
=============================================
+ Coverage      97.26%   97.67%   +0.40%     
=============================================
  Files             27       27              
  Lines           1172     1163       -9     
=============================================
- Hits            1140     1136       -4     
+ Misses            32       27       -5     

Files	Coverage Δ
core/generator/Generator.cc	100.00% <100.00%> (+7.35%)	:arrow_up: