Remove these Sentinel hacks when Click 8.3.1 ships

[simonw]

This commit: 1361ed5

Has a workaround for this issue:

• Sentinel is passed to type_cast_value pallets/click#3069

The fix for that is set to go out in Click 8.3.1 - release issue here:

• Release 8.3.1 pallets/click#3087

Once that is released, remove the nasty hacks.

[simonw]

This looks like it may be a clean revert:

git revert 1361ed571122ffd4dd8c3f1fdea52a1423422c04