mathlib3

5cafdfff - chore(algebra/group/basic): dedup, add a lemma (#6810)

chore(algebra/group/basic): dedup, add a lemma (#6810) * drop `sub_eq_zero_iff_eq`, was a duplicate of `sub_eq_zero`; * add a `simp` lemma `sub_eq_self : a - b = a ↔ b = 0`.