The function was removed, but its declaration and changelog entry about
its removal were forgotten.

The comment in the test doesn't make any sense as the function doesn't
exist anymore, so get rid of it as well.

These are not useful anymore, as we don't affect the instance's
configuration.

We no longer have any setters which affect an instance, so
`git_submodule_save()` is no longer relevant.

With this one, we can get rid of the edit_and_save test.

Similarly to the other ones. In this test we copy over testing
`RECURSE_YES` which shows an error in our handling of the `YES` variant
which we may have to port to the rest.

Moving on with the removal of runtime-changing variables, the update
setting for a remote is whatever it was when it was looked up.

This lets us specify in the status call which ignore rules we want to
use (optionally falling back to whatever the submodule has in its
configuration).

This removes one of the reasons for having `_set_ignore()` set the value
in-memory. We re-use the `IGNORE_RESET` value for this as it is no
longer relevant but has a similar purpose to `IGNORE_FALLBACK`.

Similarly, we remove `IGNORE_DEFAULT` which does not have use outside of
initializers and move that to fall back to the configuration as well.

As submodules are becomes more like values, we should not let a status
check to update its properties. Instead of taking a submodule, have
status take a repo and submodule name.

Instead of affecting a particular instance, make it change the
configuration.