Remove x,y distinct variable constraint on sbco2v
Remove x,y distinct variable constraint on sbco2v

- Remove sbequv, sbequiv, sbequif, sbequf, sbco2iv and sbco2vf. They are
  replaced by sbequ, sbequi, or sbco2v.
- Remove sbco2vALT theorem (and take its proof for sbco2v).

metamath/set.mm,metamath/set.mm,metamath/set.mm