Drop executable-availability skips from the digdelv tests

Drop executable-availability skips from the digdelv tests

The digdelv modules skipped themselves when the tool under test was
not built, but dig, delv, mdig, nslookup, host and nsupdate are
always built on the main branch, so the guards could never trigger
and would only hide genuine breakage as a skip.

Assisted-by: Claude:claude-fable-5