Remove TestExchangedata* tests for old macOS

The exchangedata() system call has never been supported on APFS
introduced in macOS 10.13 (2017, unsupported since late 2020) and since
macOS 10.14 (2018, unsupported since mid 2021) all HFS+ drives are
converted to APFS. As far as I can find you can't use HFS+ drives with
current versions of macOS:

https://old.reddit.com/r/macsysadmin/comments/tk5lsi/use_hfs_instead_of_apfs_with_monterey/
https://old.reddit.com/r/MacOS/comments/jgodi9/i_cannot_update_because_i_cannot_convert_to_apfs/

So it seems we can safely remove this test. It wasn't run in the CI
anyway, as this only tests macOS 11 and 12.