Remove HAVE_LIBNCURSES; it is required to be true

Remove HAVE_LIBNCURSES; it is required to be true

Also, it was a confusing name, since it does not necessarily mean that
we literally have libncurses; rather that we have tputs etc.

* configure.in (HAVE_LIBNCURSES): Remove; it is required to be true.

* src/dispnew.c: Remove HAVE_LIBNCURSES test;
it is always true on relevant platforms.

* nt/config.nt (HAVE_LIBNCURSES): Remove undef; not needed.