Removed mts.conf; the maildelivery option went into slocal directly.

The MTS is a non-MUA part of MH, which is/gets removed in MMH.
Now, you cannot change the global maildelivery file without changing the
code. You stil can specify -maildelivery to slocal.
I've also remove support for the env vars MHMTSCONF and MHMTSUSERCONF,
which were added by Earl Hood in 2010. We might want to add support
for them again. I need to think about the future of slocal anyway.