Remove the code darkening backfacing surfaces for shadows

[jtorresfabra]

Remove the trick that was done to prevent artifacts in backfacing surfaces due to self shadowing. A correct bias tweaking make this trick innecessary and also avoids the issue it created: backfacing surfaces were being darkened twice.

Launch Checklist

• [x] briefly describe the changes in this PR
• [x] apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'