refactor(ring_theory/free_comm_ring): don't use is_ring_hom (#13352)
refactor(ring_theory/free_comm_ring): don't use is_ring_hom (#13352)




Co-authored-by: Scott Morrison <63f04b4d96235eb042cc46d94f14d241d2a26722@gmail.com>

leanprover-community/mathlib,leanprover-community/mathlib