dmcs was noted as deprecated in its startup message script many years
ago (2016), and dmcs itself was *actually* deprecated even years before
that -- 'dmcs' itself only executes 'mcs' with the .NET 4.0 SDK enabled
anyway.
Since CMake will be the new primary build system, let's go ahead and
drop support for dmcs entirely in this setup.
Signed-off-by: Austin Seipp <aseipp@pobox.com>