sim: drop targ-vals.def->nltvals.def indirection

sim: drop targ-vals.def->nltvals.def indirection

We don't have alternative nltvals.def files, so always symlinking
the targ-vals.def file to it doesn't gain us anything.  It does
make the build more complicated though and a pain to convert to
something newer (like automake).  Drop the symlinking entirely.

In the future, we'll want to explode this file anyways into the
respective arch dirs so things can be selected dynamically at
runtime, so it's not like we'll be bringing this back.