Remove df-or and related theorems from iset.mm
Remove df-or and related theorems from iset.mm

- Remove pm2.54 and df-or, as the idea is dealt with by
dfordc, pm2.53, and pm2.54dc.
- Remove orri as it is just an inference version of pm2.54
- Remove imor as the idea is dealt with by imordc and imorr
- Adjust comments on pm2.53 and dfordc accordingly

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