selftest: Remove duplicate setup of "spn/upn namespaces" in the customdc testenv

selftest: Remove duplicate setup of "spn/upn namespaces" in the customdc testenv

The call to $self->setup_namespaces() was allways in error, as the design
is to have the in the state that it was backed up in, but before commit
08be28241b808845c4b51a4c47765a9416ca3aa7 the error return was not
checked and so this was harmless.

The customdc environment is not tested in selftest currently, as
it is intended to be used for manual testing of domains from backup
files not as an automatically constructed environment.

This makes:
 BACKUP_FILE=samba-backup-2024-04-11T14-10-20.437096.tar.bz2 SELFTEST_TESTENV=customdc make testenv
work again.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jo Sutton <josutton@catalyst.net.nz>