zfs: don't use zfs_freebsd_copy_file_range

There is one data corruption problem reported and fixed upstream, not
cherry-picked here yet.

On top of it the following fires under load:

VERIFY(zil_replaying(zfsvfs->z_log, tx));

The patch which introduced the entire machinery is a revert candidate,
but as the machinery came with a dedicated feature flag, doing so would
render affected pools read-only at best. To be figured out.

As a temporary bandaid at least stop the active usage.
Note this patch does not make the feature disappear from zpool upgrade.

Sponsored by: Rubicon Communications, LLC ("Netgate")