Cleanup ENABLE_FORGIVING_JWT_COOKIES toggle and related code

[feanil]

Once #197 has landed and we've managed to migrate to forgiving JWT Auth, we'll want to update the code here so that forgiving JWT auth is the default and the other code is removed.

• Cleanup the ENABLE_FORGIVING_JWT_COOKIES
• Cleanup code that uses it so that we retain the code that would run if ENABLE_FORGIVING_JWT_COOKIES=True

[robrap]

• We should probably create a DEPR ticket.
• Clean-up includes removing USE_JWT_COOKIE (and USE-JWT-COOKIE) as well.
  • Github search in openedx org.
  • Github search in edx org.

[robrap]

This ticket is now a duplicate of the DEPR, so I am closing this ticket.

• [DEPR]: USE-JWT-COOKIE header #371