feat(group_theory/index): Golf proof of `relindex_eq_zero_of_le_left` (#13014)
feat(group_theory/index): Golf proof of `relindex_eq_zero_of_le_left` (#13014)

This PR uses `relindex_dvd_of_le_left` to golf the proof of `relindex_eq_zero_of_le_left`.





leanprover-community/mathlib,leanprover-community/mathlib