chore(algebra/ring): delete a duplicate instance (#5215)
chore(algebra/ring): delete a duplicate instance (#5215)

In #3303 and #3296 which were merged 1 day apart two versions of the instance comm_monoid_with_zero from a comm_semiring were added 5 lines apart in the file, delete one.






leanprover-community/mathlib,leanprover-community/mathlib