Final update to SMT-COMP 2021 options (#6739)

Final update to SMT-COMP 2021 options (#6739)

This commit:

- Disables `--tear-down-incremental=X` for the competition since
  it currently does not work correctly on master and a fixed version did
  not show significant benefits.
- Changes the occurrences of `--nl-ext` to `--nl-ext=full` because it
  is now a mode option.
- Removes the use of `--bv-assert-input` because the option currently
  has some issues in incremental mode (#6738)
- Removes the use of `--bitblast=eager` for the model validation track
  because it produces invalid models (#6741)