Remove atf-ksh93 in favor of atf-sh -s/usr/local/bin/ksh93.

Shebangs are allowed to be passed arguments, and since atf-sh already
accepts a -s argument to run the test using a different shell, just use that
instead. This eliminates the need for the hackish atf-ksh93.

Submitted by: will
Sponsored by: Spectra Logic Corp