refactor(ring_theory/polynomial/symmetric): simplify proof (#6135)
refactor(ring_theory/polynomial/symmetric): simplify proof (#6135)

... of `mv_polynomial.is_symmetric.sub`

2X smaller proof term

Co-authors: `lean-gptf`, Stanislas Polu

This was found by `formal-lean-wm-to-tt-m1-m2-v4-c4` when we evaluated it on theorems added to `mathlib` after we last extracted training data.


Co-authored-by: Jesse Michael Han <3016e39019d41e617073345c1de9b12b412fb035@users.noreply.github.com>

leanprover-community/mathlib,leanprover-community/mathlib