v1.18: vote: deprecate unused legacy vote tx plumbing (backport of #274)

[mergify]

start cleaning up some plumbing

---

This is an automatic backport of pull request #274 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of b27c80a has failed:

On branch mergify/bp/v1.18/pr-274
Your branch is up to date with 'origin/v1.18'.

You are currently cherry-picking commit b27c80a920.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   programs/vote/benches/process_vote.rs
	modified:   programs/vote/src/vote_state/mod.rs
	modified:   sdk/program/src/vote/state/mod.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   sdk/src/feature_set.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.6%. Comparing base (04356e7) to head (e7712bd).

Additional details and impacted files

@@            Coverage Diff            @@
##            v1.18     #276     +/-   ##
=========================================
+ Coverage    80.6%    81.6%   +1.0%     
=========================================
  Files         827      827             
  Lines      224496   224515     +19     
=========================================
+ Hits       181015   183289   +2274     
+ Misses      43481    41226   -2255     

[willhickey]

Approved for backport to beta