Removing vestigial mistral reward model converter references

[SalmanMohammadi]

I must've missed this in #1160.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/1307

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As of commit f68672f with merge base c1fabb9 ():
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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.75%. Comparing base (f6ddfcc) to head (f68672f).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1307      +/-   ##
==========================================
+ Coverage   68.31%   70.75%   +2.43%     
==========================================
  Files         255      258       +3     
  Lines       11796    11904     +108     
==========================================
+ Hits         8059     8423     +364     
+ Misses       3737     3481     -256     

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