selftest: Remove unused environment variables

selftest: Remove unused environment variables

We only really want generic environment variables. For 2+ DC
environments, we have the $SERVER and $DC_SERVER (aka PDC) variables.
However, lots of testenvs also export really specific environment
variables, e.g. VAMPIRE_2000_DC_SERVER_IP (despite that testenv being
only used for a single test case).

Previously the <testenv>_SERVER variable was used for DRS tests, but we
can avoid the need to do this now. The other variables are not used at
all.

The RODC and TRUST environment variables are still used by a few tests.
SUBDOM_DC_SERVER is only used within Samba4.pm and not exported.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>