duplicate is_group_hom

Stream: maths

[Johan Commelin]

I think these are duplicates.
https://github.com/leanprover-community/mathlib/blob/master/src/group_theory/quotient_group.lean#L50
https://github.com/leanprover-community/mathlib/blob/master/src/group_theory/quotient_group.lean#L82

[Johan Commelin]

@Chris Hughes You can probably fix this faster than I can create a PR (-;