ZTS: remove empty zpool_add--allow-ashift-mismatch test

[robn]

Motivation and Context

Found this weird empty test that confused me. I wanted to not be confused.

#15509 added this empty test. I don't know if it was meant to have stuff in it, but it doesn't, so there's no point keep it around.

@grwilson if it was a mistake, and you do have a nice test lying around somewhere, it'd be great if you could post it and I'll close this out.

Description

Remove the test and ZTS boilerplate for it.

How Has This Been Tested?

Ran zpool_add test tag by hand, all passed (except the one expected failure, natch).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

Good find. We can always add this back in George has a version of this.